Why can't they just be bad?

I watched a horror movie last night. It was the kind with an evil child. The kind that makes you think maybe adoption or foster care is the worst idea you ever had. The little girl has a demon soul. I digress. The protagonist remarks at one point that she wishes the defendants in a case were just bad, not crazy. I understand. Why can't they just be bad?

Recent events in Arizona made me think this too. Why can't this guy, this guy who killed six people and tried to kill a lady who seems like a genuinely nice politician (do they make them anymore?), just be bad? The first thing the sheriff told the press is that the man seems unhinged. Well, if you'd just killed a kid and five other people, wouldn't you seem unhinged? Of course he's not completely sane. What did John Wilkes Booth gain for the South when he murdered Lincoln? He actually made it worse--Reconstruction under Johnson was harder than it had to be. Was Booth off his nut? Probably. A little. But does anyone say that to excuse his actions? Of course not. He was a bad man.

Psychiatry tries to make us feel better about the things people do. I think it actually makes it worse. I can't say so and so was evil because his brains were scrambled eggs. Boo hoo! So he was infected with the crazy, so what? Why can't he just be a bad guy? I think it's a way of neutralizing evil. If it can be explained by insanity, it must not really exist, right?