I understood that was the whole point they were making in the movie, but many times the aliens faught back. The whole "human nature" part was kind of silly though, all living creatures have the same natural instincts to keep their territory and to fight for self preservation. If surrounded by hostile creatures any animal would fight back, both for themselves and the group. The fact that the aliens never had fighting back in their minds kind of makes you wonder how their species even survived long enough to gain all their technology as fighting for the ability to reproduce in a safe environment is biological.
It's just a movie I know, but I love arguing about random things haha.