I think that sometimes we comfort ourselves with the notion that people like Hitler, Stalin etc were insane. As I understand it the legal definition of insanity is that the person accused did not understand the consequences of their actions, and this certainly does not apply to Hitler, Stalin, Pol Pot or any of the other leaders of the twentieth century who were responsible for genocide on a massive scale. They weren't insane, merely evil, however, uncomfortable it may be for us.