They are surprisingly willing to harm and kill others in the service of some new purpose if they are so instructed by a new authority

From “On Tyranny: Twenty Lessons from the Twentieth Century” by Timothy Snyder

The Yale psychologist Stanley Milgram, contemplating Nazi atrocities, wanted to show that there was a particular authoritarian personality that explained why Germans behaved as they had. He devised an experiment to test the proposition, but failed to get permission to carry it out in Germany. So he undertook it instead in a Yale University building in 1961 – at around the same time that Adolf Eichmann was being tried in Jerusalem for his part in the Nazi Holocaust of the Jews.

Milgram told his subjects (some Yale students, some New Haven residents) that they would be applying an electrical shock to other participants in an experiment about learning. In fact, the people attached to the wires on the other side of a window were in on the scheme with Milgram, and only pretended to be shocked. As the subjects (thought they) shocked the (people they thought were) participants in a learning experiment, they saw a horrible sight. People whom they did not know, and against whom they had no grievance, seemed to be suffering greatly – pounding the glass and complaining of heart pain. Even so, most subjects followed Milgram’s instructions and continued to apply (what they thought were) ever greater shocks until the victims appeared to die. Even those who did not proceed all the way to the (apparent) killing of their fellow human beings left without inquiring about the health of the other participants.

Milgram grasped that people are remarkably receptive to new rules in a new setting. They are surprisingly willing to harm and kill others in the service of some new purpose if they are so instructed by a new authority. “I found so much obedience,” Milgram remembered, “that I hardly saw the need for taking the experiment to Germany.”