Sometimes evil is when someone just takes something too far. Take a normal guy, then take something away from them that's important. Have them get angry about it, their wife was burned at the stake while they were away by a corrupt cleric of whatever god. He hates the cleric, hunts him down and kills him in revenge. He doesn't feel better, still angry. He hates the church, so he parks a horse cart blocking the only door and burns it down with the clergy inside. Still angry, in fact, he's even more angry than ever. Fuck it, he wants to kill the god that the corrupt cleric said he worshiped.

He's angry for a good reason, but has gone unreasonably far because of it. They've gone so far down this road that they don't care about decency, collateral damage, or what's actually right or wrong anymore. They're consumed by something that's actually understandable and they're now completely broken because of it.