Remember, too, the horror at the discovery of just who was looking the other way or was actively, eagerly participating in all this. Not idiots. Fools maybe. But not incompetents. These were trained professionals, intelligent people, those otherwise respected in their societies, before the war, publishing papers, books, which were read worldwide and studied. In that sense, at least, they were not stupid. That's the frightening lesson that Alexander wanted people to remember in his postwar-studies of The Third Reich.
I recently ran across an interesting expression, "desk murderers." Desk murderers were those who may not have personally killed anyone but they gave the orders, signed the papers, etc. all from the comfort of their offices. Every totalitarian system needs its desk murderers in order to function. Groupthink makes this possible. Groupthink enabled the killers to suspend all judgement and just focus on being efficient little monsters. Of course our schools and popular culture never teach about the dangers of groupthink.