Who really believes that such a thing would be possible, regardless of external punishment?
Surely only a psychopath could kill someone in anger and not ultimately be destroyed by the act, almost by definition, and we've already decided that those people are medical cases, and that punishing them literally does nothing.
I guess it depends on how you personally view human nature; this stuff is hard to pin down. Maybe I'm just naive.
Just think of examples of self defense, or crimes of passion (e.g. the killing of your lover's mistress). Why do we never hear of how all of these people fall into bottomless pits of depression?
Particularly in your description of it "destroying" someone, I wonder if you are thinking of "Crime and Punishment". Remember that was not a crime of passion, but a planned killing expressly chosen to be in cold blood. Killing in cold blood is a different story, but we are not talking about that.
On the subject of psychopaths, I don't think this category exists in the sense that you mean it. In particular, the personality traits usually thought of as characterizing psychopathy, do not predict criminal behavior strongly (or at all) [1]. So I think you need to reconsider your factual claim that there is a category of people, defined by certain mental traits, who are "medical cases" and cannot be deterred from committing crimes.
As you say, people have very different views of human nature. I consider the idea that a mentally healthy person would never want to harm someone else, or else would suffer from extreme guilt if they did, to be wrong. Also, most arguments for this viewpoint, including your own, seem to suffer from the No True Scotsman fallacy.
[1] http://www.psychologicalscience.org/index.php/publications/j...
Count on this. People are excellent at rationalizing their own actions.
I cannot prove this, and I personally am not inclined to believe in proofs from first principles in these matters, but I can appeal to people's gut sense of what is moral and decent. Who really believes that a person can murder someone else (and let's suppose that they were not raised in a violent or abusive environment that would tend to normalize acts of violence), out of pure malice, and then be allowed to enjoy the same happiness as if they hadn't committed that act?
I'm aware that there are people who don't really believe in "pure malice" and are therefore more sympathetic to murderers. While I'm happy to here their arguments, my own observations and experience suggest that there is tendency in human nature to do what is wrong, and this should be punished. I'm also not arguing against taking into account a person's upbringing and even their "mental health" when sentencing.