Evil person doing evil things for the sake of evil. What’s his motivation? Umm… evil. Why? Because… evil.
Evil villains, aka. the Bad Guy© doesn’t really cut it anymore for the antagonist for anything above a young child reading level. Nowadays, readers want to understand why the antagonist is behaving the way they are. “Because they’re bad” doesn’t satisfy.
That’s not to say that it has to be a socially acceptable motivation. Revenge is a popular one, for example. A murdered child could easily tip an already unstable parent into a life of hunting down the killer. Throw in a dash of conspiracy theory, and have it be a death due to a hazing at a fraternity, and now you have someone hunting down frat boys and killing them. Your hero is the cop trying to find him. Protagonist and antagonist, both motivated by an inner desire for justice.
How about a tormented childhood? He was homeless and abused as a child, so now wants to control every aspect of life around him to provide himself with a mental “security blanket.” Put him in a position of power or authority, and watch him manipulate and control everyone around him.
Both of these may not be how you or I would handle the situation, but with a bit of empathy, we can see how they can get to that point. And that’s the point.