You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat. At length I would be avenged; this was a point definitively settled…

Montresor makes this comment at the beginning of the story as he introduces his desire to get revenge on Fortunato for his insults. Montresor’s comment that anyone who knows him well would know that he was not a man to threaten portrays him as sinister and cunning. He is not a man to get mad, or even let on that he is angry, but he will get even. His statement that he will be avenged “at length” further highlights his patience.

My heart grew sick — on account of the dampness of the catacombs. I hastened to make an end of my labour.

This odd change in Montresor’s mood comes as Fortunato stops responding to his taunts. Given the coincidence of timing with Fortunato’s apparent surrender, Montresor blaming his heartsickness on the cold seems a convenient excuse. Instead, Fortunato’s refusal to put up a fight or plead with Montresor seems to have taken the fun out of the murder. Montresor’s disappointment that Fortunato does not continue to plead for mercy can be read as emblematic of Montresor’s cruelty. It’s also possible Montresor is sobered by Fortunato’s apparent lack of understanding of why Montresor wants to brick him up.