First, it's not clear to me that a persons own feelings of guilt are a sufficient punishment for their acts. A person might feel that they were at least partially justified (even if they were not), or might find other rationalizations to mitigate their feelings of guilt.

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...