Why didn't Batman just kill Joker when he had the chance?

Do you mean aside from the reason you're given straight away afterwards? Batman is the hero. He might not be a huge fan of Gotham justice but he doesn't go around murdering people simply because he feels like it...

THE JOKER : Just couldn’t let me go, could you? I guess this is what happens when an unstoppable force meets an immovable object. You truly are incorruptible, aren’t you?

Batman secures the Joker upside down. The Joker is laughing.

THE JOKER : You won’t kill me out of some misplaced sense of self-righteousness … and I won’t kill you because you’re too much fun. We’re going to do this forever.

Alright, first off, Joker is a psycho. A PSYCHO. One that goes around killing people in the most horrific ways possible. Alright. Now, look at Batman. What is he? A FREAKIN' MULTIMILLIONAIRE who has like every single thing a man could ask for on this planet. Now, since Batman has nothing better to do than just be rich, eat food, and get fat, he decides to fight crime. So, Batman is at the top of society and when you compare him with Joker, what do you get? Hmm...let's see. A person who everyone respects, is very rich, and wants keep people out of harm's way and a psychotic clown who every hates and rejects. Joker has no life. Batman has two lives (literally). Honestly, wouldn't a rich fat person like Batman feel the least bit pity on someone who is only crazy because of one surgery that went wrong? In other words, Batman is a merciful hero who sees the bad as well as the good in everyone...yes, including Joker even when he's blowing up popcorn stands. :)

The longer, more in-depth answer boils down to "Batman almost never kills anyone". Batman's entire raison d'etre is his revulsion towards killing, which began when his parents were murdered in front of him as a child. He became a crime-fighter specifically because murder was so abhorrent to him; he is a vigilante, not an executioner.

Senior Writer at Comics Alliance, Chris Sims, dealt with this subject at some length.

Q: Batman’s no kill policy: when did it start in the comics and what do you see as the limits of it? (Killing vs. “Not Saving”) — @ELB_Brian

A: ...By 1940′s Batman #4, in a story by co-creators Bill Finger and Bob Kane — which is about as definitive as you can get — Batman reminds Robin that “we never kill with weapons of any kind.”

...As far as I’m concerned, “Batman Does Not Kill” is one of the constant, immutable traits of the character, as much an inherent and necessary part of him as anything else. There are plenty of metatextual reasons for it — ranging from the nature of the super-hero as something that appeals to children to the fact that if Batman actually killed the Joker, then we wouldn’t get any more Joker stories and that would suck — but there are also equally valid in-story reasons for it.

And again, they’re often misconstrued, both by readers and by the creators. There’s a scene in Judd Winick’s run where Jason Todd confronts Batman and flat-out asks him why he doesn’t just kill the Joker — which, all things considered, is a pretty fair question — and Batman answers by telling him that it would be too easy and that it’s a slippery slope that, much like Pringles, once he popped, he would not be able to stop.

That’s nonsense.

Batman’s a guy who trained himself to be the world’s best martial artist and a guy who could solve crossword puzzles in his head while cross-referencing crime locations with Italian clown operas. He came back from a broken back through sheer force of will and beat an addiction to Venom in a weekend by locking himself in his basement and growing a beard. I’m pretty sure that if he set his mind to killing the Joker and then not committing any more murders, he could probably make that happen.

I actually like the scene up to that point and its portrayal of Jason Todd’s pretty legitimate beef with Batman’s policy, but it’s the halfhearted, wishy-washy “oh but I want to!” exploration of why Batman doesn’t kill completely tanks it for me. The only reason that Batman should give as to why he doesn’t kill is that the Batman doesn’t kill. That’s all there is to it.

But there is an underlying reason for it, and it’s one that the scene above doesn’t touch. And it all hinges on the idea that Batman is a crimefighter. That’s a very specific word that’s applied to Batman for a very specific reason, and it encapsulates the very specific aspect that separates him from other characters. At its core, the idea of Batman is one that’s extremely oppositional, and it’s set not just against evil in general, but the very concept of capital-C Crime.

It seems contradictory given that in many ways, Batman is a criminal himself, a vigilante who operates outside the law with methods and that are certainly illegal. Fleisher’s encyclopedia even includes a list of Batman’s “particularly flagrant violations of civil liberties and due process” that sprawls out over two pages. And that’s only the notable ones, in a book that was published 30 years ago.

But if we’re going to accept Batman as a hero — and I think it’s pretty clear at this point that I have — then there needs to be a clear demarcation of what separates the idea of Batman from the idea of Crime. And that’s the easiest thing in the world to figure out.

Batman’s entire idea of Crime, his entire perception of what it means to break the rules set down by society, descends from exactly one moment: his parents’ murder. That one act, the taking of a life, is the defining moment of his life, and it defines what he swears to battle against. The very act of killing another person is what he has devoted his whole life to working against, and it’s complete and utter anathema to him.

It’s these layers of symbolism and concepts literalized into characters that make Batman so compelling as a character and, and what defines his existence on a metaphorical level. For Batman, Crime is killing, and the opposite of Crime is Batman.

As to the limits of this rule, that’s a little bit more of a gray area. There’s a common interpretation of Batman as someone who just doesn’t want anyone to die, ever, and while that’s certainly a valid interpretation up to a point, I think it centers far more on the act of murder as a criminal transgression. For me, it comes down to two simple concepts that are etched in stone: Batman doesn’t kill, and Batman will not allow one person to kill another. These two rules apply to everyone, from Commissioner Gordon on down to the Joker, and as long as they’re in place, I think of the portrayal of Batman as valid. Anything beyond those is just set dressing.

The idea of “killing” versus “not saving,” is a much more metaphysical one that really comes down to whether your personal philosophy equates inaction with an evil act. The infamous “I won’t kill you, but I don’t have to save you” scene is my least favorite part of Batman Begins, but at the same time, stories where Batman does more than the bare minimum and goes out of his way to save the Joker at great personal risk always ring really false for me.

This same question appeared on Quora, and it generated some great answers too:

Put simply, Batman doesn't commit murder, because he refuses to intentionally take a life with his own hands and become an executioner. The basic answer is easy enough to articulate. But the reason behind it is very complicated.

Bruce Wayne witnessed the simple power of taking a life, when he was a child watching his parents die in front of him. The act itself is easy, something anyone can really do if they want to, but the impact of murder is complex and monumental, because the implications of an execution last forever.

Joe Chill shot Bruce's parents in a moment of fear and desperation, just to grab some money and without the intention of taking anyone's life -- but his simple act of reflexively pulling a trigger, in a split second, forever changed the world through the ripples it sent out, taking the Waynes from the world and ending Thomas Wayne's medical practice and the parents' philanthropy, and of course sending Bruce on a path inescapably toward becoming Batman.

Bruce is aware of this with every fiber of his being. He relives that murder in his darkest moments, and it is in the memory and honor of his parents that he fights to make their city a better place. He can never become the thing that struck them down, a murderer who takes the simple path that sends out those endless ripples. The purposeful taking of another human life, to assume the power and responsibility of forever ending a life, is the defining event against which Batman rose to resist. The moment he takes a life, he has lost his reason to exist, because he will have become the very thing he was born to end.

Bruce accepts that he must be a criminal, a vigilante, to do his work. But with this he accepts as an unavoidable element of his mission -- to stand against the peculiar and very special circumstances of Gotham's corruption that reaches to the highest government offices and taints the justice system and law enforcement, Batman would necessarily have to operate outside of the legal system. To be free of outside influences and accountable only to himself and his mission, too, required being an outlaw of sort.

But this lack of accountability is also a burden, and it means he must carefully weigh his actions and police himself as much as he polices the city -- and he is well aware that without any other accountability or authority, he must restrict himself from actions that go too far and that are outside the ultimate goal of standing for the ideal of the rule of law as a social contract. A man accountable only to himself might be able to excuse actions that can be corrected or amended if he is wrong, but when he is answerable only to himself, the temptation to weigh the lives of others and to deal out judgment as a God deciding the ultimate fate of victims, is too much. A line must be drawn, or there will eventually be no line at all and no limitation on his own actions, because he will stop being accountable even to himself if does not hold himself accountable for the irreversible and absolute exercise of authority over life and death.

It is easy -- too easy -- to think that yes, the Joker has killed so many victims and escaped so many times, the only way to make him stop killing innocent civilians is to just kill him. In Frank Miller's The Dark Knight Returns, there is a great moment when, after the Joker has detonated a hidden bomb in an apartment complex, Batman thinks to himself that he will stay and help the police pull people out of the rubble and do the best he can, and then he'll count the dead and add them to the list of all of the people he himself has murdered by letting the Joker live. So Batman doesn't fail to understand the horrible math involved, the terrible moral trap that presents itself the moment he begins to let himself even consider the possibility of killing the Joker in order to save future lives.

[Let me take a moment to comment on that last point, just to be clear: it is ONLY in the context of preventing future presumed murders, of course, that we can even begin to talk about the moral dilemma of "why doesn't Batman kill the Joker," because the notion of killing the Joker out of revenge or to make him pay for his past crimes doesn't enter into the equation. Punishment isn't what Batman is about, it's not his mission, it's not his mental frame of reference for what he does and why he does it. And he does on some level realize that the Joker's madness and psychosis are so absolute and pure, "punishment" itself is an irrational response to someone who exists beyond such concepts like the Joker. It is only the idea of killing the Joker to preserve other lives in the future, then, that we are talking about here.]

While Batman realizes the implications of the mathematical calculations, that refusing to execute the Joker will almost surely mean more innocent deaths if the Joker escapes again (and he always does, eventually), he also realizes that if the point is that the Joker's life isn't as valuable as the lives of his victims -- so much so that the mere CHANCE of more victims is enough to justify murdering the Joker -- then a single innocent life should be enough to justify murdering the Joker. It cannot be a case of weighing the number of innocent lives, if innocent life is so precious it justifies murder then even one should be too many, since the equation is one life (a victim's) versus one life (the Joker's).

And if one innocent life is too valuable, and would justify taking the life of a killer, then the same equation applies to most of Batman's other arch villains as well. And it applies to the mobsters. And it applies to any killers. And, if the equation is one in which the likelihood and chance of future victims is enough to justify murdering someone to stop the potential/likelihood of future victims, then rationally chronic drunk drivers and armed robbers and many others also qualify.

Which is where the lack of a prohibition means the lack of a clear line, which means the lack of ANY line, which means the lack of any accountability other than himself necessarily leads -- irresistibly and unstoppably -- to lack of accountability even to himself. He becomes absolute, and murder of anyone becomes justifiable. Because once you've justified the above examples, you place in your own hands a presumption of moral certainty (which you MUST presume, you MUST feel with absolute certainty, or you can never trust yourself to begin murdering based on who you believe should be murdered) that will eventually lead to the same certainty about your best guess, about your gut feeling, and the basic minimum standard becomes so arbitrary that there IS no minimum standard anymore. Once the idea enters your head that a person should die, your absolute moral authority, without accountability whatsoever, means you can find a reason to kill them if you wish to do so.

That is the logical progression, in the context of one man placing himself beyond all outside accountability and authority, and granting himself the latitude to judge when it is acceptable to murder other people. The mathematics is a ruse, a distraction to tempt and allow that first taste of blood, that first deceptively easy step across a line that vanishes forever once you cross it. If the loss of innocent life is enough reason to execute another life judged not innocent, then one innocent life becomes enough reason to execute another life judged less innocent.

Batman knows this, because he lives it every day -- he lives it by watching it transpire in Gotham, by fighting it when it manifests daily in the villains and in the hearts of ordinary citizens and in the minds of cops with a badge and a gun and a creeping sense that it's so easy to justify taking a life to stop future wrongdoing. But most of all, he lives it because it resides in his own mind, every single day, when he must remember that terrible moment as a child when he watched another man make the calculation of Bruce's parents' lives versus a few dollar bills. And he lives it because within his own heart, he hears that little whisper tempting him to cross the line, with the Joker or with the Penguin or with the mobsters or with a serial killer.

So he refuses to become a murderer, because he knows that murdering the Joker leads to murdering all of them, making each killing easier than the last. And that casts him as the very thing that created him, as the thing he fights against, because at that point the only difference between Batman and the Joker would be that Batman thinks he's able to justify his own murders.

From another angle:

Because the Joker wins if Batman kills him. That's what the Joker wants. Everything he does is to taunt Batman into killing him. In fact, the interesting part of their relationship, the real conflict of each story, is not to see if Batman will stop him (he will), but to watch Batman struggle with not killing him, because anyone other than Batman would of course kill him. This self-control is Batman's superpower.

The Joker and Batman are each trying to prove a point to society - and really to us, the readers. The Joker wants Batman to kill him because he perfectly embodies chaos and anarchy, and wants to prove a point to everyone that people are basically more chaotic than orderly. This is why he is so scary: we are worried he may be right. If the Joker is right, then civilization is a ruse and we are all truly monsters inside. If the Joker can prove that Batman - the most orderly and logical and self-controlled of all of us - is a monster inside, then we are all monsters inside, and that is terrifying.

The Joker is terrifying because we fear that we are like him deep down - that he is us. Batman is what we (any average person) could be at our absolute best, and the Joker is what we could be at our absolute worst. The Joker's claim is that we are all terrible deep down, and it is only the law and our misplaced sense of justice that keeps us in line. Since Batman isn't confined by the law, he is a perfect test case to try to get him to "break". The Joker wants Batman to kill a person, any person, but knows that the only person Batman might ever even remotely consider killing would have to be a terrible monster, so is willing to do this himself and sacrifice himself to prove this macabre point. Batman needs to prove that it is not just laws that keep us in line, but basic human decency and our natural instinct NOT to kill.

If Batman can prove this, then others will be inspired by his example (the citizens of Gotham, but again, also the readers), just as we are all inspired every day to keep civilization running smoothly and not descend into violence, anarchy, and chaos. This ability to be decent in the face of the horrors and temptations present all around us is humanity's superpower, the superpower of each of us. The struggle of Batman and the Joker is the internal struggle of each of us. But we are inspired by Batman's example, not the Joker's, because Batman always wins the argument, because he has not killed the Joker.

This basic logic applies to all superheroes who don't kill, but the Joker-Batman conflict is the most perfectly distilled example. There are a lot of other good answers on this page, and they are all different-but-correct ways of looking at the question, but to me, the philosophical and thematic reasons above are more resonant than the plot and character reasons that exist within the logic of the story.

Recommended reading: The Killing Joke (1988), obviously, but also Legends of the Dark Knight Annual #1 (1991, which the above picture is from, by Denny O'Neil and Jim Aparo), if you can find it. Excellent. And of course, also watching The Dark Knight. Nolan and Ledger got the Joker perfectly right.

And a more philosophical answer:

There are literally hundreds of possible explanations and reasons one could consider when attempting to unpack Batman's no kill rule. One of the more interesting analysis I've read concerning Batman's no kill rule comes from the book Batman and Philosophy in a chapter titled Why Doesn't the Batman Kill the Joker? by Kantian ethics expert Mark D. White.

In a nut shell, White discusses the differences between Utilitarian and Deontological ethics and how this applies to Batman. The argument is that the Utilitarian thing for Batman to do would be to kill the Joker as it would bring the most good to the most amount of people; not killing the Joker is perhaps even a very selfish thing to do as it places the needs of the individual (Batman) over the needs of the community (Gotham). Batman not wanting to kill because he doesn't want to "dirty his hands" is not a fair argument from a Utilitarian perspective. Interestingly this is a claim that has been made by both Hush and Jason Todd (and perhaps even by the Joker) on several occasions. However, Batman is apparently very well read on his Kant as he seems to believe that the ends never justify the means; instead the means must be a morally justifiable action unto itself. Thus Batman operates from a more Deontological ethical background than a Utilitarian one. Dressing up like a bat, scarring the living crap out of bad guys, and beating them to a pulp when need be, are all morally justifiable to Batman, but killing is not. After all, the Joker could reform tomorrow and take up origami, who is Batman to make such a call? Wouldn't killing make him no better than Joe Chill?

I would argue that the Batman books even present us with a clear Utilitarian model a la Ra's al Ghul. If Batman is our Kant than Ra's al Ghul is our very twisted Jeremy Bentham. In other words, if Batman were to remove all moral speed-bumps to his war on crime, then he would be no better than the villains he is fighting against. Ra's for example has no problem with killing, and even desires to depopulate the majority of the human race in an attempt to bring the world back to "balance." Unlike with Batman, the ends always justify the means, no matter how brutal.

Wouldn't the logical and Utilitarian means for Batman to clean up crime in Gotham be to simply blow up Arkham Asylum? Perhaps so, but that would require Batman to cross an ethical and moral line he is not willing to cross. And it's for that reason that we call Batman a hero and Ra's al Ghul a villain.

The subject, unsurprisingly, comes up frequently in the comics, and Batman's answer is usually more or less the same:

Finally, the phenomenal graphic novel The Killing Joke deals with this as follows:
Batman says to the Joker:

I've been thinking lately. About you... About me. About what's going to happen to us, in the end. We're going to kill each other, aren't we? Perhaps you'll kill me. Perhaps I'll kill you. Perhaps sooner. Perhaps later. I just wanted to know that I'd made a genuine attempt to talk things over and avert that outcome. Just once. Are you listening to me? It's life and death I'm discussing here. Maybe my death... maybe yours. I don't fully understand why ours should be such a fatal relationship, but I don't want your murder on my hands.

The conversation continues in the final pages:

Another reason that Batman won't kill the Joker that nobody is talking about: They live in a world where magic is real. In the DC Universe death is temporary. Jason Todd was brought back to life because a version of Superman had a temper tantrum and "punched" reality. Bruce Wayne brought Damian back to life by stabbing his corpse with a magic crystal. All it would take is somebody like Black Hand to bring Joker back as a Black Lantern or Klarion the Witch Boy to bring him back as an immortal demon. Joker even once tricked the Spectre (God's red right hand) to give him his powers! For all the death Joker brings to Gotham, Batman keeping him alive is the safest thing he can do.

In the Dark Knight, Batman is in a more realistic world where Joker is probably not going to escape custody. People forget that Joker mostly escapes prison because it is a comic book. Heck, Joker once escaped with a hot air balloon in the comics:

This would be a lot harder to pull off in the more realistic world of the Dark Knight where Joker might go to a maximum security prison or somewhere like Guantanamo Bay prison. So basically, in comic world Joker will escape if he doesn't stay dead. In movie world, Joker can be kept in prison like a real-life domestic terrorist or insurrectionist anarchist trying to harm people.