“They are so sure they are serving the cause of righteousness”
That’s an assumption we make FAR too often. You don’t know that.
True, in theory. However, very few people intentionally do what they know to be evil for the sake of evil.
They do evil because they believe it (incorrectly) to actually be good, because they believe they are justified (often for revenge for evil done to them), or because they believe the evil they do is justified because it is in the service of a greater good.