What does "In pace requiescat!" mean?

In pace requiescat is a Latin phrase which means "May she/he rest in peace." In Poe's classic short story, Montresor vows to get revenge on Fortunato for a "thousand injuries" and elaborates on how he planned and executed the perfect murder. Montresor goes on to reveal how he manipulated Fortunato to follow him into the depths of his family's catacombs, where he eventually shackled Fortunato to the back wall and proceeded to bury him alive. After Montresor forces the last stone in place, he mentions that no mortal has disturbed the bones for a half a century and ends his narrative by saying, "In pace requiescat!" (Poe, 5).
One could interpret the traditional Latin blessing in two ways. Readers could infer either that Montresor made the statement sarcastically or that he sincerely meant what he said. Given the fact that Montresor feels remorse while burying his enemy alive, which is revealed when he confesses that his "heart grew sick," one could infer that Montresor sincerely meant the Latin blessing. The phrase also corresponds to Montresor's feelings of closure. Now that Montresor has got revenge and successfully carried out his murder, he no longer has any ill feelings towards Fortunato, which influences him to repeat the Latin blessing.

Montresor knows that his revenge against Fortunato would not be satisfying if he himself were caught and punished. He must also realize that killing Fortunato would not bring him satisfaction unless it enabled him to get rid of the bad feelings he harbors against the man. Montresor feels anger, hatred, resentment, and humiliation, among other things. When he achieves his perfect revenge and his hated enemy is dead, Montresor feels cleansed of all his bitter feelings. Revenge is sweet. After the passage of fifty years, Montresor can feel assured that he will never even be suspected of causing Fortunato to disappear so mysteriously. If he thinks of Fortunato at all, he pictures him as a wretched skeleton still chained to the granite wall in the narrow niche, still wearing the soiled rags of his jester's costume. He no longer hates this man at all. Montresor has achieved closure. He has cleansed his heart and mind of all the bitterness he felt because of the thousand injuries he had endured. This is why it is appropriate that Montresor ends his story with the words "In pace requiescat!" or "Rest in peace!" Montresor is not being sarcastic. He really and truly means it. This is sweet revenge. Fortunato is nothing now. He is of no more importance to Montresor than all those other bones in the catacombs. Montresor feels obliged to say "In pace requiescat!" Who else could say it? He is the only person in the whole wide world who knows where Fortunato's body is interred, and the only person in the whole wide world who knows how it got there. Montresor wishes Fortunato to rest in peace because he himself has found peace of mind by disposing of him.