What does the quote "A wrong is unredressed when retribution overtakes its redressor" mean?
At the beginning of "The Cask of Amontillado," Montresor specifies his ideas of the perfect revenge. One requisite is that the "redressor," that is, the perpetrator of the revenge plot, should not suffer any kind of "retribution." The two main forms of possible retribution for Montresor would be to have the police authorities arrest him for murder and to be found guilty, or to have Fortunato's many powerful friends and relatives suspect him of being guilty of Fortunato's disappearance and take private, personal action against him. No doubt if they had strong suspicions they would subject him to torture to make him reveal the location of Fortunato's body and then cut him to pieces.
Montresor guards against suspicion by pretending for years to be Fortunato's good friend. Montresor is the last person in the world anyone would suspect of being involved in a crime against Fortunato because it is so well known that the two men are the best of friends. Throughout the tale Montresor keeps addressing Fortunato as his friend, his poor friend, and his good friend; and he refers to him in the same terms. This is because he has forced himself to think of his enemy as his friend in order to be able to maintain a totally friendly manner toward him. Montresor has so conditioned himself to calling Fortunato his good friend that he keeps doing it from force of habit.
Poe's entire purpose in creating these details is to leave the reader assured that the revenge was perfect. Montresor had to think about the future. It wasn't enough just to wall Fortunato up and leave him to die. Montresor had to be totally above suspicion for years after the event, because there would be investigations and discussions of this strange disappearance for a long time, during which Montresor would have to act just as concerned and just as mystified as everybody else. We see enough of Montresor in the story to feel assured that he will be able to play his part to perfection.