Montresor has some very specific criteria for exacting successful and complete revenge. In the first paragraph of the story, he shares his belief that he "must not only punish, but punish with impunity." In other words, he must not only carry out his revenge on Fortunato, but he must do it in such a way that Montresor himself does not reap any negative consequences. Therefore, we can probably safely assume that, had his crime been found out, he would have been tried in a court of law and sentenced to death; in this case, then, he would feel that his revenge was incomplete or unsuccessful.
Thus, he would probably become angry and perhaps even try to figure out a way to avoid his punishment. After all, with Fortunato dead, there would be no second opportunity for Montresor to achieve the revenge he so desires. His pride, I think, especially given the quotation above and his family's own motto, would compel him to try to avoid punishment in order to make his revenge an ultimate success.
Remember, though, that Montresor does seem to be confessing his story to someone who "know[s] the nature of [his] soul," as he says in the first paragraph. He feels, perhaps, that he cannot be punished at this point, some "half of a century" after he committed the crime.