If somebody kills a man in order to get revenge for something that man did to him, and then if the killer gets caught and either gets sent to prison or executed, doesn't the killer still have his revenge? Poe seems to think the killer has to get away with the crime without even being suspected, or else he has not gotten the revenge he is after. Yet there have always been many people who have committed revenge-murders and gotten convicted and who probably still felt satisfied that they had gotten the vengeance they desired. There must be many cases in which the person wanting revenge actually gets killed himself in the act of killing his enemy. I can't think of examples in literature (or in the movies, either) but there must be many examples. One example that comes to mind is Hamlet, who is killed indirectly by Claudius and then kills Claudius while he himself is dying. Doesn't Hamlet get the revenge he has been wanting all throughout the play? Another example that comes to mind which might be even better than Hamlet is Humbert Humbert's murder of Quilty in the novel Lolita (and in the film adaptation). Humbert not only gets caught but doesn't even try to avoid capture.

Montressor clarifies that statement by saying, "I must not only punish but punish with impunity." He just doesn't want to get caught or exposed as the person who murders Fortunato. He is eminently successful because Fortunato completely disappears. He continually refers to Fortunato as his friend and his good friend because he doesn't want anybody even to suspect that he knows anything about Fortunato's disappearance.

"Redress" means "to set right" or "to rectify"

"Unredressed"--- the opposite of above.

"Retribution" means "punishment that one deserves for a wrong he has committed"

Montressor is saying that Fortunato has not "paid" for having wronged him, so Montressor is justified in taking restitution for the "insult" done to him.

This phrase was spoken by Montresor in the opening lines of the story as he tells the reader about the wrongs committed against him by Fortunato. The word "unredressed" means not right or not made up for. What Montresor is saying here is that the wrongs committed against him by Fortunato were never made up for or Fortunato never made the wrongs right so now retribution or punishment will have to serve the redresser, Fortunato, instead. He is basically saying that Fortunato has to pay and since he hasn't taken the time to make things right, he, Montresor will have to now take matters into his own hands and issue Fortunato a fitting punishment. Montresor is a very unreliable narrator because he is secretly scheming this revenge for some wrong which he never names and his revenge plot is more than a bit extreme.