"The Cask of Amontillado" is all about Montresor's revenge: the terrible revenge he wreaks upon the hapless, misnamed Fortunato. We're never told exactly what Fortunato has done to Montresor that warranted being walled up alive inside the catacombs. But whatever it was, this unspecified insult has driven Montresor into a frenzy that can only be satisfied by some serious payback—the most terrible revenge you could possibly imagine.
In his warped imagination, Montresor feels that revenge is not just necessary, it is the right thing to do. Revenge is like a moral imperative for Montresor, something he simply has to do if he is to redress the "thousand injuries" that Fortunato has heaped upon him over the years. But if Monstresor's going to gain revenge, he needs to do it properly. That means making sure that he gets away with his crime. His elaborate revenge plot has clearly been carefully planned and worked out to the last detail. This is to ensure no one will ever suspect Montresor of having carried out this wicked, dastardly deed.