What is it about Montresor that makes him an especially effective enemy to Fortunato?
Montresor claims that he has borne a "thousand injuries of Fortunato" but he never says exactly what they are. He adds that it is an insult that sends him over the edge and propels him toward thoughts of revenge. And this is severe revenge: "I must not only punish, but punish with impunity."
But even though Montresor is filled with anger and a thirst for revenge, he never lets these hateful thoughts known to Fortunato. He coldly informs the reader that he continued to be friendly to Fortunato and to smile but that "he did not perceive that my smile now was at the thought of his immolation." This is Montresor's first strategy. He acts as if he and Fortunato are still good friends and therefore, Fortunato will never expect anything sinister from Montresor.
Montresor also knows Fortunato's weakness: his pride in being a connoisseur of wines.
When Montresor greets Fortunato at dusk during the carnival, he is friendly and then starts to lure Fortunato by telling him he has just bought a cask of Amontillado but is not sure it is genuine. He further tempts Fortunato when he says that since Fortunato seems busy, he will summon Luchesi to taste-test the supposed Amontillado. Fortunato insists that he is the better judge of wine and insists on going with Montresor to judge for himself.
Using reverse psychology, Montresor continually tries to talk Fortunato out of accompanying him to test the wine. He says that he doesn't want to be a bother to Fortunato and that he doesn't want Fortunato to get sick in the damp vaults where he stores the wine. These friendly protests only make Fortunato more determined to test the wine. As they descend into the vaults, Montresor keeps feeding Fortunato drinks to dull his senses and make it easier for him to continue manipulating Fortunato.
Montresor continues to alternate these strategies. He is friendly and appears concerned for Fortunato's health. But he keeps him drunk and keeps using Fortunato's pride to lure him deeper into the vaults.