Because the story is entirely about revenge, a thesis statement could cover any number of possible topics, from potential consequences to the debate over exactly what "injury" Fortunato enacted on Montresor. One good theme is the correlation between pure revenge and Montresor's warped sense of morality and justice. Montresor explicitly states that his revenge is both justified and should not be considered a crime, even though he understands that it will be a criminal act.
At length I would be avenged; this was a point definitely, settled -- but the very definitiveness with which it was resolved precluded the idea of risk. I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser.
(Poe, "The Cask of Amontillado," croads.virginia.edu)
Despite this, he goes through with his plan without a second thought. Montresor therefore thinks of justice as a moral balance, with "sides" between people capable of being tipped to one side or another. Since he assumes that no other will avenge him, he must take revenge without having another "injury" (such as imprisonment) done to him in turn.
An example of a thesis using these themes would be: "Montresor's revenge is, in his mind, entirely moral according to his ideas of personal honor, but he still understands that his act would be seen as wrong in the public eye."
Montresor believes that "A wrong is unredressed when retribution overtakes its redresser." In other words, revenge is not truly exacted if the person seeking revenge is punished for it. Therefore, a crucial part of Montresor's revenge lies in maintaining the appearance of innocence so that he is not brought up on charges of murder. To be so charged would negate the value of the act in his eyes. However, Montresor never seems to consider that, though he may succeed in maintaining the appearance of innocence, he may still feel guilty about his crime, and this guilt could be punishment enough to "overtake" him as the redresser he discusses.
When Montresor speaks, we can see that he is speaking to someone. He says, "You, who so well know the nature of my soul, will not supposed, however, that I gave utterance to a threat." Who is this "You"? Who would "know the nature" of Montresor's soul? Some would argue that he is speaking to a priest as a priest would, theoretically, have this knowledge. Further, in the story's conclusion, Montresor says, "For the half of a century no mortal has disturbed [Fortunato's bones]." Therefore, it has been some fifty years since he took his revenge on this enemy. It is reasonable to suggest that Montresor is now an old man, perhaps on his deathbed, and that he is making his final confession to a priest before his death. The final line of the story ("In pace requiescat!"), which translates to "rest in peace," could be applied, then, to either Fortunato or to Montresor. Perhaps Montresor felt that he could not rest in peace with this sin still on his conscience, and so he had to confess it. This would mean that his conscience has been weighted with his guilt for the past fifty years, and, if this is the case, has he really gotten away with his revenge? Wouldn't his guilt be a terrible punishment?
Therefore, a potential thesis could read: Although Montresor believes that his revenge on Fortunato was successful because he escaped punishment for his crime, ultimately, his own guilt punishes him for what he did, and this negates the achievement of his revenge.