Montresor is clearly acting with malice aforethought; he has taken steps in advance to set up an elaborate plan for Fortunato's death. However, since he is telling the story, and since he is an unreliable narrator, the reader has only his word that he is committing a justifiable act.
A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
(Poe, "The Cask of Amontillado," eNotes eText)
In other words, he plans for Fortunato's death in a way that cannot come back to harm him. This shows a very logical and detail-oriented mind, not one to commit murder for passion or on impulse. While it is certainly possible that there was no reason for the murder aside from revenge, there is also no evidence that Montresor is lying; the murder has been undiscovered for fifty years, and in retelling the events he has no reason to make up a story. He cannot be punished now, and so he has little reason to be anything but truthful; the story is, in essence, a boast, and so Montresor would take greater pride in telling the truth.
It could be contended that Montresor has other motives for wanting to kill Fortunato besides revenge. The following is the third paragraph of Poe’s story quoted in full.
He had a weak point -- this Fortunato -- although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine. 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.
This may suggest that both Montresor and Fortunato, though aristocrats, earn money in the ancient, decaying city of Venice by dealing in expensive items such as oil paintings, jewelry, antiques, and gourmet wines. They are competitors, and Fortunato’s much-discussed “thousand injuries” may have all been injuries in business dealings. For example, Fortunato may have outbid Montresor for an oil painting and then sold it to a British or Austrian millionaire at a handsome profit. Fortunato is well connected and far more likely than Montresor to know about impoverished upper-class Italians needing to sell off family treasures in order to survive, as well as being more likely to encounter wealthy foreigners able to pay cash for them. So Montresor may be hiding from himself the knowledge that in disposing of Fortunato he would be eliminating his chief competitor.
Montresor is telling his story fifty years after the fact. During that half-century he may have enjoyed a prosperity that was denied him while his friendly enemy Fortunato was alive. We might detect a certain smug complacency in Montresor’s narrative. We might even imagine him enjoying a glass of genuine Amontillado while remembering his successful crime. Fortunato had outsmarted him many times while he was alive, but Montresor outsmarted him on that one memorable evening at the height of the carnival.