A truly great movie. Like most really good sci-fi, it helps us look at ourselves (and our treatment of others) better. Neil Blomkamp is South African, and it's no accident the movie is set in South Africa.
Definitely interesting that you thought it might be about humans being oppressed by aliens at the beginning :)
A good sci-fi series for you to watch (though I think only watching the first season is best) is Westworld. Like this movie, it helps us examine ourselves through different eyes.