It only takes one person for something evil to occur. For example, one of the reasons many authoritarian countries haven’t changed their regime already is that the vast majority of people live in fear of the handful of people who would commit evil on behalf of the leaders.

This is a question of power. If every single person in the country realized that they only have power because they can get other people to do bad things, the leaders would no longer be in power. 

The flipside of that is that it only requires one person being willing to harm or kill another for these people to be able to keep their power. 

It’s contagious—one person begets another person willing to commit harm (or too scared to refuse). Pretty soon, a tiny minority of people grows who are willing to commit evil to keep this one person in power.

Because not everyone says no, the minority rules, and the majority seems powerless. As such, the people who are in the majority must seemingly be willing to face death at the hands of the minority to effect change.