Many people have called Montresor an "unreliable narrator." Some seem to think Montresor is telling his story to them. There are multiple ways to look at this issue, but one can definitely make the case that Montresor is a reliable narrator. Poe makes it clear from the beginning that this is a confidential communication to one single individual whom Montresor has known for many years. He addresses this person as "You, who so well know the nature of my soul." If he is unreliable, he is not intentionally so. If the reader fails to understand something Montresor says or writes, that does not mean the confidant, or confidante, does not understand it perfectly. If Montresor is "unreliable," it can only be because he does not understand himself; he is not trying to deceive the confidant or confidante, and there is no one else he could have been trying to deceive.
So, where is the evidence of unreliability? Most people point to the thousand injuries plus an insult. Montresor is certainly unreliable in what he says to Fortunato, but he knows he is being intentionally deceptive. Why does it seem unreliable to say that he received a thousand injuries from his victim? Surely the recipient has heard from Montresor a number of times over the fifty years before receiving this strange confession. If the reader does not know the nature of at least some of the injuries, the person who reads the confession must surely have heard about some of them in all that time. If they had not been corresponding for fifty years, Montresor would not be so sure he could trust the other person with this incriminating revelation. He considers the other man, or woman, a person who knows all about him, who knows the nature of his soul. Is this also supposed to be unreliable? Was Montresor making a big mistake in trusting this other person with such information? That seems unlikely because he had taken such great precautions, not only to commit the murder, but to do so with what he calls "impunity." No one would suspect him of having anything to do with Fortunato's strange disappearance because everyone thought they were the best of friends. Montresor repeatedly refers to his victim as "my friend," "my good friend," and "my poor friend" even when he is leading him to his death. He has conditioned himself to think of Fortunato as his friend and always to speak of him as his friend, even as his good friend. He should be above suspicion, and he would have been the one who showed the most concern about Fortunato's whereabouts for the longest time. The reader can only call Montresor "unreliable" if he assumes Montresor is addressing him, but Poe has invented a narrative device that enables him to leave out a plethora of background information in order to focus on the dramatic aspect of the narrative.