I suppose there are the occasional blackhearted people who do bad things because they are bad things to do. But the vast majority of evil acts... Well, first of all, the vast majority of harmful acts are the result of carelessness or cluelessness, not even malice. And then among the malicious acts there is a small fraction that could truly be called evil. But every single one of those comes from doing the right thing.
Which is Jason's problem with this: If Sarah can just elect herself as a mouthpiece for God, discounting anyone else's disagreement because of the purity of her faith, then how could she stop him, for example, from pretending that he's got Jesus Christ on the phone. The modern viewpoint being that of course, since God and Jesus are both imaginary, they shouldn't be telling people what to do anyway. Of course, that's true, but if you are trying to do religion that way, you're doing it wrong anyway, and you should shut up about your religion because all you're doing is fucking everything up. But more importantly, we tend to create the Gods we require, from the parts we find lying around.
I would only trust a God that pushed back, you know? Like how you never want to get so rich that people would say Yes to you as you turn into Michael Jackson: That's the kind of God you know is fake, the kind that is cool with whatever you are doing. But more importantly, you don't have to talk about God to see how true this is. If everything you do is for good reason, which it is when you are doing it, then you are always working toward the Good, no matter what you do. You are putting your words in the mouth of God, or morality, or whatever guides you, which is a disaster every time.
Tenderness leads to the gas chamber, is how O'Connor put it: The idea that once you substitute your own morality for one that benefits everyone -- identifying yourself with God, the same way an addict does -- you can justify everything and anything. And everybody wants to feel like the good guy, which means anything you do is a good thing (at least while you're doing it), which means that every monster got there by saving the world.
Most of the time, by taking something they felt weird about and trying to make sure nobody ever had to feel weird about it again. Saving everybody else from their own problems. I mean, just the words on the page look pretty heroic, imagine what a person actually undergoing that amount of ego inflation would feel like, thinking about that. "All these weak or prurient people who can make up excuses as to why vampires shouldn't be knocked out. They will thank me one day, when they're safe again. Or they won't. But either way, I have to save the world."