How do I kill all mobs in a world without being in it?

I just disabled mobs by setting it to peaceful mode and commands, now the slimes are gone!

This is a bit of a complex way to kill the slimes, but it technically should work.

1. Set your render distance to one chunk. By doing this, you will be able to move slightly faster while doing everything after this, as it won’t render all the slimes.
2. Place down a command block. Make this command block a repeating one and enter this command: /kill @e[type=!minecraft:player]and add a redstone block next to this command block.
3. Turn up your render chunks. By turning up this, you will be able to kill all slimes in your visible area. Then, destroy the command block summoning all the slimes.

Source: I tested it myself for good measure.

There are multiple ways to remove all slimes:

1. Set your render distance to one chunk and set entity render distance to the lowest value possible. By doing this, you will be able to eliminate temporary lag since it won’t render all the slimes.
2. Load the world and set the gamemode to peaceful
3. Change everything back to how you prefer it!

Or:

1. Set your render distance to one chunk and set entity render distance to the lowest value possible. By doing this, you will be able to eliminate temporary lag since it won’t render all the slimes.
2. Load the world and type /gamerule doMobLoot False. (To stop slimes from dropping loot)
3. Type /kill @e[type=minecraft:slime] and repeat it at least 3 times. (To kill all slimes with different sizes)
4. Type /gamerule doMobLoot True to re-enable mob loot.
5. Change everything back to how you prefer it!

And finally you could try:

1. Setting your render distance to one chunk and set entity render distance to the lowest value possible. By doing this, you will be able to eliminate temporary lag since it won’t render all the slimes.
2. Load the world and type /tp @e[type=Slime] ~ -300 ~ to teleport all slimes into the void. poof!
3. Change everything back to how you prefer it!

And if all of those in-game solutions fail you, there's this program called MCedit which can do all kinds of stuff including removing the commandblock and removing all slimes. However, development seems halted since 2016 so it might not work.

Source