There is, of course, the very first line of the whole story, which says, "The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge." We do not get to know the specific nature of the insult, but it is clear that Montresor found the insult to be bad enough to warrant murdering Fortunato.
There is also: "At length, I would be avenged; this was a point definitively settled..."
"I must not only punish, but punish with impunity."
"I continued, as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation." As is necessary for plans of vengeance, the person who is planning their revenge would not want to let their target grow suspicious of their intentions.
"I had told him that I should not return until the morning..." This is followed by Montresor explaining that he had ordered them to remain at his home for the night, knowing full well that they would do no such thing, that they would leave as soon as he was gone. So he returned to his home with Fortunato, knowing that no one would be there, which was perfect for his plans.
Finally, there is the start of the actual act of imprisoning Fortunato: "A moment more and I had fettered him to the granite." This begins the end of the story, when Montresor walls Fortunato into the recess in the wall in his family catacombs, which is the ultimate act of revenge.