Evil is a problem, and I don't mean the 'Problem of Evil'. The problem has to do with the fact that the usual modern sense of 'evil' is something more like 'a calculating desire to cause unjustified harm to others', and that explains only a tiny fraction of the unnecessary suffering in the world. When people do each other wrong, it is much more likely to be because they are scared or careless. Sometimes people are vengeful for some perceived slight - convincing yourself that someone deserves vengeance is an easy way to justify doing things to them which would otherwise be wrong. Another way is to convince yourself that someone just doesn't deserve your respect because they aren't good enough, or human enough.
Scared people are not really calculating. Careless people don't want to do wrong. Vengeful people tell themselves their actions are justified. When someone dehumanises their victim, they are not really counting them as others. In their own heads, almost nobody is evil. Hatefulness and disdain for others can fall within the definition I gave earlier, but even that depends on your perspective in any given case. This makes it interesting to me that there are so many fictional antagonists whose perspective we are not really expected to understand. They are evil, so of course the good guys need to fight against them; their ideas about why it would be okay to do what they do are completely opaque to us. The most interesting thing about this is that the 'evil' party here is painted as inherently hateful, barely even human. You may sense a kind of circularity here.
The idea of the heartless antagonist seems to have so much narrative pull that it is almost irresistible to treat it as if it helps make sense of everything. I see people wasting a lot of energy doing little more than fighting against personifications of evil, when the real problem is usually that people are given the opportunity to do something that is good for them but bad for someone else, and they just do it without thinking about what it means, or they think it's okay because the other person was an arsehole anyway. There is a lot of subtlety there, and subtlety is boring and difficult, so even though we all know that our opponents are probably just people who have picked up wrong ideas, or bad habits, or a perspective we don't agree with, it is so much easier to think of them as just being bad guys.
I am bothered about this because I think it feeds so much of the partisan political theatre we see in our legislatures and media, so many of the wars that we are asked to support, so many of our least useful struggles. It is easy to feel like you are fighting the good fight when you work to tear down people who are abusing positions of power, but when they are just the latest figurehead of a complex power structure, it is all too easy to leave the underlying power structure intact. It is also frighteningly easy to get caught up in somebody else's game of demonisation, or we would never have needed the word genocide.
I guess what I am saying here is that the idea of evil causes great evil. Maybe some people really are just awful, but even if they are, it is not so much the people that we need to fight against; it is the awfulness, and the opportunities to express it.
Hayao Miyazaki is good on the concept of evil; here's a comic of some of his thoughts on the matter.