You can't explain behavior by merely focusing on internal characteristics of a person such as being sadistic and cruel. Such behaviors were not exactly predisposition. They were heavily influenced by external factors like commands from a legitimate power such as Adolf Hitler. The Milgram experiment was actually inspired by the harsh acts of Germans during the WWII. People can actually go to extreme lengths, even if such acts are an attack of one's personal conscience, since they believe the legitimacy of a higher power, consequently making their actions legal or morally right even when it is not.
