Montresor has not atoned for his sin. Far from it, he has not even made the slightest attempt to atone, and he tells the whole story in a boastful manner, focusing on his own cleverness in gaining his revenge.
At the end of the story, Montresor reveals that it has been fifty years since the events he has just related took place. This suggests that he may be on his deathbed, after a long life during which he has not thought much about Fortunato. If this is the case, the tone of the narrative makes it clear that he is not sorry for what he has done.
It is primarily Montresor's cool, detached attitude to his actions that sets him apart from other narrators of Poe stories, such as "The Black Cat" and "The Tell-Tale Heart." These men may not have atoned for their sins, but they have at least suffered guilt, or something like it. Montresor is unusual in the perfect equanimity with which he contemplates his crime.