Montresor is proud, cunning, envious, potentially sadistic, hypersensitive (like Poe himself) about his poverty and precarious social position, eaten up with repressed hatred and anger. He is also two-faced. The persona he presents to the world is not his true character. He acts like a courteous gentleman and is especially friendly to the man he hates the most, Fortunato. It might be added that Montresor possesses great patience. He plans to murder Fortunato but is willing to wait for years if necessary for exactly the right opportunity. A persona technically is not the same as one's real character. All of us, according to some authorities like Jung, invent a sort of mask, or persona, which we present to the world as our real selves. Part of the interest in "The Cask of Amontillado" is the contrast between Montresor's persona and his hidden self.
Montresor had to be extremely sadistic and vindictive in order to do what he did to Fortunato; but he had to do what he did to Fortunato in order to create the "effect" Poe wanted to create. The reader may be shocked and horrified at the cruelty of Montresor's revenge, but, after all, this kind of cruelty is hardly unprecedented in history. Montresor states as a simple fact without any explanation:
In its surface were two iron staples, distant from each other about two feet, horizontally. From one of these depended a short chain, from the other a padlock.
Who knows how many other wretches had been chained to the granite wall over the centuries and left to die of starvation? This was a common way of disposing of people throughout the Middle Ages. Every nobleman worthy of the name had his dungeon and his oubliette. What Montresor was doing to Fortunato was only a recent revival of a long tradition. No doubt Montresor enjoyed behaving in the manner of his noble ancestors--and this may have been what prompted him to write his long memoir about the execution fifty years after the fact.