Theory I: The Banality of Evil (from Hannah Arendt)"Hilter and the Nazis were extremely skilled at using propaganda and a deliberately gradual process to make the isolation, segregation and ultimately the killing of Jews seem "normal". The Germans who carried out unspeakable crimes were ordinary people who simply accepted the conditions of their context as "normal", "unexceptional" and "the way things are done" From Eichmann in Jerusalem.
Theory II: Conformity and Obedience (from Christopher Browning)
"To break ranks and step out, to adopt overtly nonconformist behavior, was simply beyond most of the men. It was easier for them to shoot. . . If the men of Reserve Battalion 101 could become killers under such circumstances, what group of men can not? From Ordinary Men
Theory III: Anti-Semitism (from Daniel Jonah Goldhagen)
Not economic hardship, not the coercive means of a totalitarian state, not the social psychological pressure, not invariable psychological propensities, but ideas about Jews that were pervasive in Germany and had been for decades, induced ordinary Germans to kill. . .systematically and without pity. From Hitler's Willing Executioners.