I'm first-generation American (USA) of German parents. For my entire life, I've never been able to reconcile how an entire nation could go so murderously insane, based on the relatives I've met (to the best of my knowledge, none of them was directly involved, although I don't absolve that generation of what happened).
There was a famous study, probably cited somewhere here, where people would continue to seemingly administer stronger and stronger electrical shocks to someone that provided wrong answers, no matter how much they screamed. The real subject of the test was the person administering the bogus shocks. The reasoning they gave for continuing was because they were supposed to, which sounds a lot like the excuse of "I was just following orders" given by many complicit in the Holocaust.