Its because of two things : pride, and they believe they're the good guys. Without humility, you can't see your errors.
Discussion
I don’t buy that villains think they’re the good guys. I think they try to evade or rationalize their villainy, but their conscience knows what they’re up to.
I’m with you on pride though. That ego is a monster maker.
You start off the monster. Humility is learned when you fall in some way - ego death. Without that event, no humility is learned. But people trick themselves into believing they're good - if they emulate the right people, who are usually authority figures, then they think they must be good. That's just a copy, and usually a copy of a copy of a copy etc. To learn humility, they would have to it from the source, which is a hard knock. Thus, the people who wear their goodness like a garment are, in fact, the most monstrous. If they were good, then they wouldn't need the garment, and then the good appear to be evil to the evil ones, who put so much effort into the appearance of goodness. That's why the most pious people are the most hateful, and the people highest in authority are the furthest from the good. That's the trend, at least.
Very good points - especially the first one that our inexperienced selves can be monsters in need of taming.
We need to be childlike. Maybe that's taming... A child makes no assumptions about what they experience. The hardest people to teach are the ones that already "know" - if what they know is wrong, then they have to forget before they can learn. Its much better to be child-like, unknowing and receptive in nature instead of by decision. Only children in that sense are capable of experiencing reality.
evil people simply don't do "i'm not God"