This 2008 TED lecture addresses a subject that I think is very fascinating and has a great number of social implications. It asks if the question of human cruelty is an individual or a systemic trait. It also presents the idea that the antithesis of cruelty—heroism—can be taught.
I took a course last year on the Holocaust and have always been perplexed why ordinarily moral people can be so thoroughly corrupted.
Watch the TED lecture on the psychology of evil here.