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.