Montresor is not confessing but writing a description of an event in his life of which he seems to be proud. The fact that he has waited fifty years to tell anyone about it only is intended to demonstrate that he has gotten away with a perfect crime. Many people express doubt that he really received any injuries or insults from Fortunato. These same people often believe that Montresor is making a verbal confession to a priest and seeking forgiveness for his sin. They call Montresor an "unreliable narrator." If so, then nothing in the story can be believed. Maybe he didn't even kill Fortunato. But if he didn't, then why is he confessing to it? Maybe Montresor is making up the entire story. But it is Edgar Allan Poe who made up the story, isn't it? Did Poe make up a story about a man who made up a story about an event that didn't actually happen? Why not read the story the way it is written? Why not assume that Fortunato injured Montresor approximately a thousand times and then ventured on insult? If we question Montresor, we can end up questioning every story written in the first person, including Huckleberry Finn and Moby Dick and The Catcher in the Rye.
Montressor does not actually confess to the crime he has committed and we the reader do not even know if the story he tells is true because he is not a reliable character or he is not credible. He is so distraught with his feelings of vengeance and a desire for revenge that we do not know for certain that he actually went through with it. The narrator only tells us that for half a century the "rampart of bones " went undisturbed. He never says for certain that the reason they were disturbed half a century later was because he confessed. Furthermore, if Fortunato was buried alive he was buried in the catacombs which were actually the tombs of the family of the house so discovering a body among a mass tomb would not be suspicious.
Montresor was very specific in the beginning to explain that Fortunato had "wronged him" and deserved punishment. However, in order for that punishment to be fair and just to the insults hurled upon Montresor, it had to be done secretly This secret, as Montresor says, is "not only punish, but punish with impunity''; that is, to punish Fortunato without being caught or punished himself. Therefore, he can not confess to the crime. We can assume that this confession is being done towards the very end of his life, perhaps even on his deathbed, when not punishment could be exacted upon him.
In addition, Montresor is an unreliable narrator. He is obsessed with injuries that he does not even describe in detail, suggesting perhaps that those injuries were not as damaging as his violent act suggests. The lack of remorse he shows 50 years later, coupled with the passioned explanation of his actions that he makes, help to support the idea that this is an unstable man run amuck with perceived insults, and not a sane man dealing out deserved punishment.