Why does Montresor seek revenge on Fortunato, and why does he say, "A wrong is undressed when retribution overtakes its redresser"?
In Edgar Allan Poe's "The Cask of Amontillado," the narrator, like so many of Poe's narrators, is unreliable in that he does not provide any reason for his revenge other than the vague statement,
The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult,I vowed revenge.
Then, ironically, the narrator assumes that readers know him:
You who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat.
Obviously, then, there is a great deal of ambiguity about Montresor's "revenge" that he feels (1) he must seek and perform with impunity as well as (2) receive acknowlegement of this revenge on the part of the victim. Both of these goals of revenge are attained: Montresor walls in Fortunato in the tomb/catacombs, and Fortunato is well aware of what Montresor has done as he calls to him, asking to be allowed to return to the carnival, and finally crying "For the love of God, Montresor!"
We do not know why Montresor wants to have revenge on Fortunato. We are told that Fortunato has "injured" him in a thousand ways. But we also can see that Fortunato does not fear Montresor or act strange around him. So that implies that Fortunato does not know Montresor is mad at him.
The phrase that you cite means that there you do not really get revenge if you get caught and punished for doing it. So Montresor does not think it is real revenge to kill someone if you get caught and punished for the murder.