Plead guilty vs. plead insanity

Here's something I've never fully understood. When a person commits a horrific crime, why on earth do they spend so much time debating over whether they're crazy or not? I guess after someone kills their kids you have to decide whether to put them here (jail) or there (hospital) for the rest of their lives, but that's sort of a small detail in the grand scheme of things, don't you think? Once they've established guilt (in the sense that it was this particular person and not someone else), then that's it. Even if you were sick and you didn't mean to do it, you can't blame anyone for thinking that you just might do it again. Which is, of course, why we lock people up in the first place. So either way, sick or no, it's curtains, lady!