District 9: Original Hostile Intent?

Very doubtful

Yes, the Prawns are far more advanced, but there is very little showing that they have malicious intent

1. The ship doesn't seem to have any capital weapons or smaller vessels that could be used to harass the opponent.

2. The Number of weapons is insufficient - if I remember correctly, what we see is at best an equivalent of a rifles (which indeed can be a mining tools like @JohnMeacham mentioned). Yes, there are more powerful than what humans have, but any defensive army would have a gigantic numerical superiority, that would have to be countered by enormous firepower which aliens don't have: the few advanced rifles wouldn't do much against a regular army equipped with standard rifles (not counting tanks, planes, artillery etc...).

3. The Prawns are not an army : For an army to be efficient they need to actually follow the orders of their superiors. Yes, the leadership can die, but under each general there are few Majors, then Captains, Lieutenants all the way to Corporals and Privates. If the one on the top is incapacitated, someone replaces him. Of course, it is possible that Prawns fight just using the "horde" style (as in - just kill everything that moves and screw the orders!), but for that they are far too docile.

Imagine that you could go back in time and want to conquer, let's say, XIX Great Britain, but you could take only one ship. You would take a battleship (or even better - a carrier), packed it with disciplined soldiers armed to the teeth. If you were like the Prawns from the movie, instead of a carrier you'd have an old cargo ship and instead of soldiers you'd have a group of refugees. A few smuggled M-16 and AK-47 wouldn't do much against the disciplined army, even equipped only with a single-shooting rifles.