What is the meaning of the phrase, "A wrong is unredressed when retribution overtakes its redresser"?
Montresor takes a lot of pride in explaining (or over-explaining) the means, reasons, and effects of his revenge. In this phrase, Montresor is saying that a wrong is not really avenged if the punishment (retribution) controls or overtakes the redresser (Montresor). What is meant by "overtakes"? I think it means one of two things. The wrong is not really avenged if the retribution becomes more important than the person exacting the retribution. In other words, Montresor wants to punish Fortunato, but he wants this to be about himself and Fortunato. The other, more accepted explanation of this line is that the redresser should not get caught. This is what is meant by the retribution taking over the redresser. If the redresser gets too involved with the retribution, he may forget the overall strategy. He may get careless and get caught. He will be overtaken by the retribution. Montresor seems to be saying that revenge will not be as satisfying if he gets caught himself. So, he vows to remain in control of the retribution.
This is why Montresor chooses to bury Fortunato. He essentially hides all evidence of his crime. In this way, he completely controls the retribution and the evidence.