I would not say that Montresor is a psychopath. I think he understands exactly what he is doing and, far from being unstable, is cold, cruel, and calculating in exacting revenge for an insult. His personal concept of honor is seemingly very pre-modern, rooted in the concept that one's reputation is a precious thing, and that insults are not to be tolerated. On the other hand, the requirements of such a moral code would seem to dictate that one ought to achieve satisfaction publicly, as a way of saving face. Montresor, obviously, does not gain satisfaction in this way. In any case, I do not read him as insane. Evil, perhaps, but not a psychopath.

1.  Since Poe never reveals more, the reader can only guess at what the insults or injuries may have been. I have always thought it was probably some sort of slanderous statement(s)--an insult(s) of some sort.

2.  Not completely reliable, in part because of his biased hatred for Fortunato and because of his mental state.

3.  People still resort to murder to settle personal matters, but few plan it as successfully as Montresor.

4.  Montresor appears to be telling his tale on his deathbed or shortly before his death. Had he bragged about it earlier, his secret may have never been kept.