We see in films what we are drawn to seeing. In Children of Men (aside from the technical brilliance of the cinematography), I see the ways people treat other people as less than human. Sometimes it is for a cause, and humans are deemed expendable to that cause's ends. At a larger scale, it is because one group of people is considered outsiders, different, Other--and there appears no reason to treat the Other as human.
It is not all there is to see in this very good film. But it is something that is there.