Conformism

In the sixties, Stanley Milgram, a psychologist of Yale University made a series of experiments on conformity. He tried to assess the degree of obedience to authority by persons. His experiments were both simple and powerful. He pretended to be experimenting how memory is affected by electric shocks. He asked volunteers to pose a series of questions to a person that was hidden by a screen. Every time the person gave the wrong answer the volunteer was asked to push a button that would give the person an electric shock of increasing power, up to 450 volts. The person hidden by the screen was an actor and was not receiving any shock, naturally, He was nonetheless instructed to scream as to show pain and to beg to be freed from the experiment.
Unexpectedly the majority of volunteers (more than 65%) continued to press the button and inflicted the maximum voltage, just because they were asked to do so by a Yale professor. So powerful was the obedience and conformity inducted by authority.

It is very much probable that certain terrible episodes in the human history like the persecution of Jews and Roma during WWII and on a very different scale the My Lai massacre during Vietnam War, the Abu Ghraib prison and Guantanamo Bay scandal in modern times can be explained with the results of Milgram experiment.

Human beings are not bad or good by nature, but social structure, chain of command and control mechanisms can have dire consequences in some cases.