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.
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.