Poe very wisely starts his story with the words "The thousand injuries of Fortunato..." If we accept Montresor's statement that he has been injured a thousand times, as I believe we should, then his desire for revenge seems justified. A thousand injuries is a lot of injuries. As far as the "insult" is concerned, I think this is only included because it triggers the desire for revenge for the thousand injuries. When Fortunato ventures upon "insult," it shows that the man is going to get even more overtly injurious. Montresor has, as he says, borne the injuries as best he could. Now it would seem that Fortunato wants to see if Montresor will tolerate open insults.
What a strange pair these men are! Why does Montresor stick around to be injured a thousand times? It must be because he is somehow dependent on Fortunato. There is strong suggestion that both these men are in the same line of business. They buy and sell one-of-a-kind valuable articles. This is suggested by the following quote:
Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially;—I was skilful in the Italian vintages myself, and bought largely whenever I could.
Fortunato is rich. Montresor is poor. No doubt Montresor sometimes borrows money from Fortunato or asks him to become an ad hoc partner in a purchase. For example, an impecunious aristocratic Venetian family may have to part with an oil painting by a great master in order to survive in their deteriorating palazzo in the decaying city of Venice. If Fortunato and Montresor are business associates, and if Fortunato usually takes unfair advantage of Montresor in their deals, this would explain the thousand injuries and explain why Montresor puts up with them.
The other action Montresor could have taken would have been to break off all relations with Fortunato. We are all at liberty to do this when we find that someone we know is not treating us with proper respect--and this is what we should do. However, it may be that Montresor simply cannot afford to break with Fortunato. They appear to be "friendly enemies." They cooperate and compete. This Luchesi, who never actually appears, is another friendly enemy.
If Montresor simply stopped having anything to do with Fortunato, his friendly enemy could become an unfriendly enemy. Fortunato might be able to ruin Montresor socially and financially. Montresor says of him:
...although in other regards he was a man to be respected and even feared.
Montresor is afraid of him. We have all known people like Fortunato. They can be bad friends and worse enemies. It is best not to get involved with such people in the first place--if we will trust our first impressions!
Since Montressor never tells the reader what wrong Fortunato committed against him in Edgar Allan Poe's "The Cask of Amontillado," we can never know if the punishment fit the crime. However, in a civilized society, murder is never a proper retaliation, so I would say it is not a justifiable act. It was a perfect crime, however, so Montressor's continued freedom must have satisfied him immensely. Punishing or ridiculing Fortunato without killing him would have left a living witness to contact authorities, so Montressor probably felt he had no choice but to eliminate this possibility. Fortunato's insult or crime against Montressor may not have been an illegal one, so contacting the authorities may not have been an option.