In the beginning of the story Montresor writes his prescription for the perfect revenge. He does not say so, but presumably the revenge could only give complete satisfaction if it were executed perfectly. In the story he relates how he did in fact carry out the perfect revenge. He is writing about his crime a full fifty years after the event. Since he has never been caught or even suspected, his revenge was perfect. The perfect revenge should give the avenger what is so commonly called "closure" these days by lawyers and journalists. Montresor has wiped the slate clean. He has freed himself from the tormenting feelings he had about Fortunato--the envy, the hatred, the humiliation of having to put up with his slurs and jibes. Since he has experienced this closure, he no longer hates his victim but may have actually begun to love him. Love can turn to hate, and often does. But hate can turn to love, especially in Poe's world. When he says, "Rest in peace," he really means it. He has buried Fortunato among his own revered ancestors and regards him as part of his own family. What Montresor wanted was not so much revenge as release from the poisonous feelings that were tormenting him.
I suspect that Poe was making up a story to get rid of his own personal feelings of hatred for one of his many enemies. Poe's financial and social situation was very much like Montresor's. Poe had nothing but poverty and misfortune after being disowned by Allan his foster father, and he must have had to suffer many snubs, cutting remarks, and other impertinencies. Rest in peace may be the most important word in the story.
I think that Montresor is talking more about himself than Fortunato. He wants to let the whole thing go. He does not want to think about Fortunato any longer. He is very self-centered, and I don't think he regrets what he did there.
This statement is ironic in my opinion, called verbal irony, because I found it sarcastic that he would utter these words when he had killed this man. Very strange, indeed, and a sign of a mentally disturbed person!
I agree with reply #2. The Latin phrase does mean "rest in peace". I hadn't thought of Montresor's putting his grievances finally to bed, but wishing his "friend" whom he has just bricked into the wall, and the bones of the unknown person(s) which he places in front of the wall eternal peace makes sense.
The statement that Montresor makes means rest in peace. In my opinion it has three specific meanings. First, he is replacing bones that have not been disturbed for 50 years, so he says the phrase to allow these bones to go back to their eternal rest.
Second, Montresor is, ironically, wishing Fortunato to rest in peace, odd, since he has just walled him alive in a tomb.
Third, Montresor is finally saying rest in peace to his 1000 undefined grievances against Fortunato. His revenge complete, he can now lay to rest his dispute with this enemy.
"He hurriedly finishes the wall, placing the pile of bones in front. Lastly, he states, "For the half of a century no mortal has disturbed them," ending with, "In pace requiescat," or "May he rest in peace."