Aword or phrase used in informal conversation is?

in the line from the end of the story "the cask of the amontillado," by edgar allan poe, the narrator has put the last stone on the wall, thus sentencing fortunato to death. just like that montresor associates his feeling of sickness to the humidity of the catacombs, and seems to be glad he has taken revenge on fortunato. however, montresor's last words are "in pace requiescat," which mean "may he rest in peace." as a result, it can be inferred that he hides or does not want to assume his guilt or remorse for killing fortunato in such a horrible way.