The Fountainhead by Ayn Rand:
“The worst thing about dishonest people is what they think of as honesty,” [Gail Wynand] said. “I know a woman who’s never held to one conviction for three days running, but when I told her she had no integrity, she got very tight-lipped and said her idea of integrity wasn’t mine; it seems she’d never stolen any money. Well, she’s one that’s in no danger from me whatever. I don’t hate her. I hate the impossible conception you love so passionately, Dominique.”
I thought of a related point:
The worst thing about confused, ignorant people is what they think of as understanding. They don't understand stuff (not even close), and that somehow meets their standards of understanding, and they stop trying to understand more.