This is an important word which serves to define the things we ought not to do. I choose to accept Philip Zimbardo’s very studied definition;
Evil is intentionally acting or causing others to act in a way that dehumanizes, harms, or kills innocent people.
The top two reasons people or groups become evil are the delegation of authority and othering.
Nazi soldiers tried for war crimes often said they “were just following orders.” This is the number one way people become evil according to Zimbardo’s research.
The number two way is Othering or using dehumanizing language to excuse evil actions. For example, “I hate stupid people” doesn’t really mean anything, and it is an easy way to give yourself an excuse to say or do any horrible thing to another person.
There is a REALLY GREAT interview of Zimbardo by Tim Ferriss which explores his decades of research into exactly how and why people or groups of people become evil versus heroic under pressure. I encourage everyone to listen to this important interview and try very hard to avoid being evil. If you would like a TL;DR, I gave a speech about it at Sierra College.