someone said, "even mentally unstable people need punishment" so they don't do it again or something like that.
first, there's little actual evidence that prison accomplishes this on most people, regardless of their stability. really. if you doubt this, it's easily researched.
second, what about someone who's schizophrenic, having delusions and hallucinations, and kills someone because they mistakenly believe that person is, say, satan and if they are let to live, thousands will die?
(and btw, schizophrenics are more likely to be victims than the other way around -- that's easily researched too -- it's just the only example my brain will cough up right now).
are they going to prison, with no psych treatment? probably. will they learn from that to "not do it again"?
would they (and others) be better served by treatment (assuming it was good treatment, which as someone else noted, is certainly not a given) that got the hallucinations to stop? yes. would that be more likely to ensure they wouldn't kill someone again? yes. more humane? yes.
i could say much more. i could note that most serious crimes are either crimes of passion or crimes of desperation -- neither of which are conducive to someone stopping first and reflecting on the fine points of law, the calmly weighing the pro's and con's. maybe for small stuff, but not big stuff.
of the few who have different motivations for serious crimes, these are: they're a psychopath (ie, they have no apparent conscience and a different emotional wiring than most humans). it's well known that consequences don't modify their behavior much. then, martyrs: people throughout history have been known to do some very radical s**t -- including murder and suicide -- for what the perceive as a noble cause. again, threat of jailtime isn't going to be an issue for them. and then, habit and custom. what? i'm talking about, say, someone born into a crime family. it's their upbringing, its' what they know, it's just -- to them -- ordinary life. is the fact that people can go to prison going to make them renounce their entire upbringing? doubtful.
but the majority of people are not very smart. they don't observe carefully, or read what studies show, or even look closely at their own actual motivations. they'll just keep believing that the death penalty deterrs people, and/or prison does, and so on, rather than learning things that actually work better and thus improving things for a whole lotta people.
plus, people simply like revenge, and they don't like things that look unfair. tho really, "fairness" is a human construct. is it fair that some are born beautiful, smart, powerful, and rich? and others are born fearful, crippled, dumb, bad-termpered, mentally unbalalced, and so on? not by human standards it's not.
i'm a big, huge fan of what *works* over what people like to *imagine* works when all they have to do is look around to see it doesn't. why? because i'd rather live somewhere nicer. nicer than what i see around me.
(note to self: yeah, good luck on that)