Um, considering that it's only attempted in something like 2% of court cases and works only a fraction of the time (a psychologist must testify that they are not mentally accountable for their actions, and even then it's unlikely the defense will work), it's hardly a "safety net". Furthermore, being judged insane does not mean you necessarily don't know what you did. It could be the inability to control what you did or, as with children who commit crimes, the inability to distinguish right or proper behavior from wrong. For instance, in the case of sociopaths, while they rarely are allowed to plead insanity because they are, for the most part, highly functioning people who few feel any sympathy for, they lack empathy, among other things, meaning that they have no reason but the chance of punishment to not hurt others. And, as they're often self-destructive anyway, that's not much of a deterrent. And, even if they successfully use the insanity defense, they don't just get set free. They will, most likely, spend the rest of their lives in mental hospitals that, in all likelihood, are worse than prison would have been.

Sorry for the rant, but it upsets me when people act like the insanity plea is an excuse or get-out-of-jail-free card.