You should be careful when you nurse a grudge. The grudge can eat you up until you end up doing something regrettable. Montresor may not have really reached that point, but he did do a terrible thing whether he realizes it or not.
The main moral question that this story raises is simply this: Does vengeance equal justice, or is revenge always just? The answer, in this teacher's humble opinion, is that "The Cask of Amontillado" shows us the sometimes deranged mentality driving acts of revenge.
Our narrator here is not Batman, nor is he even "The Punisher," but rather someone who has taken a slight offense and has applied to it the ultimate pricetag, death. Clearly we see that his state of mind, though cogently explained, is askew at best.
Hence, the moral lesson we may derive from this short piece is simply: Revenge is not always equal to the offense it portends to make right, particularly if the act of retribution is murder. A comparison one might draw is that of Robin Hood -- Was it okay for him to rob from the rich to give to the poor? Or is thieving always thieving just as murder is always murder, no matter what the motive? Such are the universal questions and lessons literature uses to keep our humanity in check.
There may be many morals to be drawn from "The Cask of Amontillado," but the moral which seems to pervade the whole story and to make it seem universally applicable might be expressed as this: We should always be very careful about how we treat others. It is very easy to offend another person inadvertently, and a single offense by word, or act, or even a look, can damage a relationship irreparably. Other people are just as sensitive as we are. That is why the Golden Rule expressed in countless languages, is stated in the New Testament as: Do unto others as you would have them do unto you.
Fortunato is obviously a boorish person who tramples on other people's feelings. He doesn't realize what he is doing. He thinks he is a funny fellow, a jester. But there are overtones of malice and cruelty in his words which reveal his character. Other people may not show that we have offended them by some thoughtless jibe or question or comment, but they remember--just as we may remember some such impudence for a lifetime. Montresor is an extreme example of how a person might remember injuries for years and retaliate, but he is just an extreme example of human nature.
To do unto others as you would have them do unto you is not just being virtuous but it is being prudent. What happened to Fortunato in this story can be seen as a payback, not just for the thousand injuries he has inflicted on Montresor, but for all the injuries he has inflicted on countless others. His misdeeds finally catch up with him in a horrible manner. It may be significant that Poe has Fortunato say:
“For the love of God, Montresor!”
and has Montresor echo his cry:
“Yes,” I said, “for the love of God!”
Fortunato seems to be thinking about his sins for possibly the first time in his life, and Montresor seems to be implying that his victim is being punished because he did not think about his sins until this moment of truth.
Poe explores the sinister mind via the character of Montressor, whose thirst for revenge consumes him. This quest for revenge leads him to plot, to plan, Fortunato's murder. Revenge can never lead to goodness. However, more significantly, Fortunato's fondness for wine--which can be interepreted as the deadly sin gluttony--leads him to his death. He willingly drinks the wine that kills him. Revenge and gluttony have no spiritual benefits, and in fact contribute to one's eternal downfall.
In my opinion, the first lesson we can learn is to never trust an unreliable narrator! The narrator of this story is clearly mentally ill, so he is highly unreliable. We never know what his nemesis did to Fortunato to make him want to murder him, so this leads me to believe that Fortunato did nothing and that the narrator was simply jealous of him. Morally, in all seriousness, we can clearly see in this story that, obviously, murdering someone is absolutely and unequivocally wrong and that revenge is a temporary fix for bigger issues within ourselves (insecurities, etc.).