In general, you can fit the motivations for evil into one of four categories: there's evil for instrumental reasons, evil for ideological reasons, evil as a res

In general, you can fit the motivations for evil into one of four categories: there's evil for instrumental reasons, evil for ideological reasons, evil as a result of threatened egotism, and evil as a result of pure sadism. There are some genuinely sadistic people, but they tend to be few and far between, to the point where once we find one of them, they spawn books and movies and huge long wikipedia pages. There are plenty of people that will trample others to advance their own selfish goals. They know that they're doing something wrong, but they feel they need to in that circumstance, or are just willing to do that. Then there are people who are genuinely thinking that they're doing good, and that the world will be a better place after they do whatever evil thing they do. In this context it's better to think of evil as a social concept rather than a personal one, because these people don't actually think they're doing something evil in a long-term context-included sense. I think conflation between some of the above types of evil is causing a lot of needless dispute in this thread. (This is mostly from the Baumeister chapter in The Social Psychology of Good and Evil, which is a very well-edited collection of notable psychologists discussing this very topic, and I highly recommend it.)