One prominent theme in the movie District 9 is the dichotomy between alienation and empathy, and how that dichotomy plays out in human morality.
The movie is narrated in documentary style, interspersing interview snippets between live-action scenes. Wikus, a police officer who patrols the alien district, initially views the aliens as totally other. He sees no similarity between himself and the "prawns," and this mindset allows him to mistreat the aliens. In the first few scenes of the movie, he attacks one prawn, kidnaps the offspring of another, and spits in the face of a third.
When Wikus becomes infected with the virus that slowly turns him into a prawn, he begins to empathize with the aliens. He no longer views them as totally other. This shift is most dramatically demonstrated when he risks his own life to help one prawn escape the South African authorities.