Poe writes in the first person to put his reader inside the head of the narrator. The rhetorical effect of this choice is for the reader to suspend, for a moment, his judgement: rather than dismissing Montresor as insane, we are forced to enter into his mindset. In this case, the point of view emphasizes Montresor’s hatred of insults. Significantly, Poe does not explain the nature of Fortunato’s transgression. Instead, Montresor explains the nature of vengeance: “I must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.” His calmness and calculation are akin to the deliberate manner the old man is observed and killed in “The Tell Tale Heart,” but the narrator here lacks the “nervous” quality of the narrator of “The Tell Tale Heart.” In that story, the purpose of the narrative was to argue that the (obviously) insane speaker was not crazy; here, the purpose is different – Montresor speaks from a position of power and condescension, and his story serves as a example of his family motto – “Nemo me impune lacessit” or “No one provokes me with impunity.” By the end of the story, the reader’s horror at Fortunato’s fate stands in stark relief to Montresor’s final words: Rest in peace.
By writing in the first person, Poe allows the reader to be inside Montressor's thoughts and witness firsthand the inner mind of a murderer. Probably the most significant element is the reason Montressor gives for killing Fortunato-- an "insult". Because Montressor does not elaborate, the reader never does find out the nature of this "insult"; however, given the fact that Fortunato willingly goes to the catacombs with Montressor, it couldn't have been very significant in Fortunato's mind. If the insult was as petty as "I don't like your taste in wine", it becomes all the more horrific that Montressor would resort to murder for something like that. If one insult could set him on the path to murder, it might leave the reader wondering if he ever repeated the crime for some other "insult".