Seriously, he convinced (how hard was it) a whole lot of people to do his bidding. The methodical coldhearted cruelty, and the sheer numbers boggles the minds of even the hard corps.
Was there that much evil to go around? were they brainwashed?
I think there inherantly is the capacity to do good and evil in the mind, and in some (weak minded? starved for power? ) the tipping point towards evil is close.
"Methodical" is the key word. Couple that with "obey or die", plus a good dose of "everyone is doing it", and most people will do anything, with little remorse, just to do their job, collect a paycheck, and go home at the end of the day.
Evil behavior can be surprisingly mundane. When made methodical and commonplace, few people would _not_ do it.
Hence the axiom that humans are inherently evil - very little encouragement is needed for most people to cross the line.