Montresor suffers many "injuries" at the hands of Fortunato, but only when he feels that Fortunato insults him does he vow revenge. Montresor is a man of wealth and station and cannot forebear insult. He feels that he must only only punish but "punish with impunity," which has a rather different meaning than the American version: he means he must do it in such a way that he will never be caught and punished.
He catches Fortunato, his "friend," during "the supreme madness of the carnival season." His speaking of him as his friend is disingenuous, and doing this during the carnival season is the height of mad brilliance: Fortunato is celebrating, like all around him. All is chaos, exemplified by the fact that Fortunato wears motley; that is, he wears a joker's outfit, complete with varied colors and bells on his hat. To die in such an outfit is an added indignity. He challenges his "friend's" connoisseurship of wine by suggesting that another man's is just as good, thereby ensuring Fortunato's compliance in his plan; he knows Fortunato will go to great lengths to prove his superiority as a sommelier of Italian vintages.
He presses the "compassion" of his friendship by noting that Fortunato has a cold and that going into the vaults would only exacerbate his condition, knowing full well that Fortunato is now committed. Every step fo the way, he is overtly concerned for Fortunato's safety, cautioning him to watch his step as they descend into the crypts, and after hearing Fortunato cough, speaks highly of him and urges him to turn back, because there is still Luchresi who can help him determine whether the cask of wine he purchased is truly Amontillado. Of course, Fortunato responds that he shall not die of cough, and Montresor responds, "True, true...." He is, as he has noted before, smiling inside at the difference between his meaning and how Fortunato understands him.
Fortunato notes that his vaults are huge, and Montresor reminds him that he has a "great and numerous family" whose motto is "Nemo me impune lacessit"--"No one can harm me unpunished." We may be assured that Montresor is deriving a great deal of pleasure in the double meaning of his words.
Fortunato points out that he is a mason--that is, a Freemason, a fraternity of men who uphold a system of morality. While the Freemasons were originally built on the craft guilds of stonemasonry, and the trowel is one of their symbols, when Fortunato asks for a "sign" to indicate that Monstresor is also a Freemason, he is asking for a hand sign, a secret way of proving he is one. Instead, Montresor produces a trowel from his clothing, which Fortunato takes for a bad joke. It is a good joke to Montresor, of course. :)
At the end, when he is almost finished walling up Fortunato, he holds up his torch to see how Fortunato is faring. Fortunato screams and Montresor's first instinct is to tremble; he is, after all, in a spooky place and momentarily unnerved. Then he pulls out his sword and pokes about in the enclosure until he realizes that this is not what he planned; no one would discover them and no one would hear Fortunato's cries for help. Thus, he himself screams as loud as an louder than his victim, mocking him. He is, without a doubt, sadistic in the extreme.