We learn that Montresor feels that he has been Fortunato's victim over and over again, but something about this particular time is Montresor's last straw. He uses the word "insult" as opposed to the thousand "injuries" that came before. From here, we know that the narrator will seek revenge. Consider the following passage, when he speaks directly to the implied reader:
I vowed revenge. 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 is ironic, because the reader is just now meeting this character, so they do not, in fact, know anything about the speaker. However, he has just told us that he is careful and that his revenge will be scrupulously planned and drawn out.
He then goes on to say that he will not get caught, implying that revenge is not true revenge if one is caught. Additionally, the revenge is no good if the person the "avenger" is trying to harm does not feel the force of the revenge. So far, he has been nothing but kind to Fortunato, biding his time and smiling, all the while harboring this secret hate and hatching a plot against him.