Why do people commit cruelties on their fellow human beings? Because they're politely told to:
For another recent movie trailer in the same genre, see The Stanford Prison Experiment.
NB: I posted this in haste and didn't make it clear why the themes of this movie are of interest here on The Interpreter. The basic answer is that the movie raises interesting questions about the motivations of those involved in atrocities. See the excellent Wikipedia page on Daniel Goldhagen's Hitler's Willing Executioners for another example of this type of discussion.