Ah now I disagree with you there. People who are "nut cases" aren't detered by anything, because they are not rational. Neither is it just to execute them, because they literally don't understand what they have done wrong. The people we are seeking to deter are those who quite clearly and cold-bloodedly work out that killing someone is worthwhile and/or profitable to them.
I agree, nut cases aren't deterred by anything because they are not rational. I cannot think of any reason why we should keep such people in live storage. We simply cannot afford keeping such people alive in those expensive dormitories they call prisons. I don't care if they can help it or not. If they are genuinely guilty of a horrendous crime, given the chance for appeal, then that's it. It's over.
Just because they are not rational does not mean they are stupid. I personally know a psychopath, she has no conscience. She truly does not understand what she is doing wrong. (she is sub-criminal) Yet she knows the difference between right and wrong. She does not know this because her conscience tells her what's right and what's wrong. She's learned the difference through life's experiences. Example: She knows cheating on her husband is wrong. That's why she goes through great lengths to conceal it. The guy is totally ignorant, she has no remorse.
And in my opinion I see no difference between a nut case or those that “clearly and cold-bloodedly work out that killing someone is worthwhile and/or profitable to them.” None of such people have the same degree of conscience as the rest of us or they simply would not be working on plans like that.