Because there will be those who wanted power, who wanted great stuff for themselves at the expense of others. Those who would go to any extend to get what they want and at whatever cost, even if it meant having to dissect one of our own. (Well, he might not had been fully one of our own by then but he was still very much so in heart, in mind and perhaps 90% in body.)
District 9 is one thought provoking film that knows how to stir up our emotions; disbelief, horror, humor, fear, pity, even shame. And it creatively breaks away from the Hollywood-led alien genre where usually major cities are targeted - think New York or Washington D.C. Also the visuals were great. That alien mother ship hovering in the city backdrop looked and felt remarkably real that it kinda scared me. It was so, well so alien. And the aliens themselves were scary too but at the same time it was a lil' funny to see how 'humanized' some of their behaviors were.
Sadly toward the end of the film, the documentary framework had to be discarded because it no longer made sense. You couldn't have a cameraman following a wanted man, could you?
Oh, and the next time these prawns come back and declare war on us, we only have ourselves to blame really. Till then, we can only live on with the image of one of them crafting a flower out of metal...