Many readers have questioned whether Montresor is crazy. If so, then they guess that he really did not receive the "thousand injuries" he describes as the narrator of his story. But in that case, maybe he did not even kill Fortunato. That, in fact, is the actual truth, since the story is a work of pure fiction. Even the cask of Amontillado did not exist.
Here is an interesting view by an expert which might shed some light on Montresor's state of mind.
"For nearly 20 years now I have maintained that there is no such thing as mental illness .... it is implicit in this view that there can also be no such thing as psychiatric diagnosis, prognosis, or treatment." Thomas S. Szasz, M.D., The Myth of Psychotherapy, American Journal of Psychotherapy, 28 (Oct. 1974): 517-526
The entire article by Dr. Szasz is accessible on the internet. He was a severe critic of Sigmund Freud and the whole idea of psychoanalysis.
Assuming hypothetically that there is no such thing as mental illness, then Montresor cannot be mentally ill. and therefore he must be telling the truth. Murderers can escape punishment by the law if they can convince a jury that they were victims of "temporary insanity," but perhaps Dr. Szasz is correct. Perhaps all murderers know exactly what they are doing and are not insane but simply unable to control their violent emotions.
Montresor hated Fortunato enough to kill him in a horrible way. But how can Montresor be described as an unreliable narrator when he is not writing about the crime while in the heat of passion but writing about it a full half-century after he committed it? Can he be unreliable without being insane? Or can he be insane without being unreliable? If he had been "insane" at the time he committed the murder, isn't it conceivable that he would have recovered after living for another fifty years and had then become a reliable narrator? At least he sounds as if he is completely cured of the terrible hatred he felt for his victim.
People do not need psychotherapy in order to recover from what is called "mental illness." It has been demonstrated that as many people recover spontaneously without psychotherapy as recover with psychotherapy and that those who recover do so in the same length of time. Even granting hypothetically that Montresor was insane when he lured Fortunato to his death, that does not prove Montresor was insane fifty years later when he wrote his letter to his confidant or confidante.