What is the major theme of The Cask of Amontillado?

murdering someone is absolutely and unequivocally wrong and that revenge is a temporary fix for bigger issues within ourselves (insecurities, etc.).

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?

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.