They forget all the little things they do that are cruel--exclusion, gossip, bullying, pointed remarks, prejudice. Anything they themselves aren't tempted to do, they call "insane", as though they couldn't possibly understand why--despite that they do lesser versions of those very things themselves.
Fact is, there's evil in every human being; and I think a lot of people like to deny it. Problem is, the second you stop fighting against it, you give in to it, and before you know it you're hurting people without caring about it. If I didn't have a conscience, I'd make a decent criminal. So would a large portion of the population, depending on how good they were at planning. There's nothing very "insane" about fulfilling your own wants; there's nothing very insane about acting on emotion; and if you don't care about hurting others to do it, then you end up with evil. It's human nature, nothing more, and it's not mysterious and unknown.
I do wish they'd stop using "madman" and "insane" and whatnot to describe evil. It's such a huge fallacy... the second somebody sees you acting weird, they think you're going to pull out an axe and go after them.
Is it really so comforting to delude yourself into believing that people can't possibly make horrible choices or do terrible things unless they're insane? It mightn't be such a big issue if every self-respecting insane person in the world weren't suffering from it!
I don't have an axe; apparently, in order to be a card-carrying "insane" person, I will now have to go out and buy one. Oh, and I need to practice my diabolical laughter, design a few death traps, and invest in a castle rigged to mysteriously collapse in the event of my inevitable defeat.