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.

In the first two paragraphs of "The Cask of Amontillado," readers learn that Montresor feels that he has been treated poorly by Fortunato a "thousand" times, but that something else has recently happened that is so significant that he can't let it go.  As Montresor narrates, he tells the reader that he did not threaten Fortunato, but that he decided that he would inflict harm on him, and that he would do it in a way to make Fortunato would know it was him, Montresor, doing it--and why. Montresor's plan is to exact his revenge without putting himself at any risk of being found out or punished, because if he were caught, the revenge would be compromised.  He smiles at the thought of what he will do to Fortunato. 

In the first paragraph, Montresor mentions that he vows to enact revenge on Fortunato for the ambiguous "thousand injuries," which he has not forgotten. Montresor goes on to say that he kept his plans for revenge a secret so that no one could suspect him of harming Fortunato. He also plans on punishing Fortunato with impunity, because if he were to be arrested or punished for harming Fortunato, his plan for revenge would not be perfectly executed. Montresor also mentions that Fortunato must be aware that he is being punished for causing Montresor a "thousand injuries." In the second paragraph, Montresor says that he gave no reason to Fortunato to doubt his good will and smiled in his face whenever they met, in order to avoid suspicion. Overall, the reader learns that Montresor has precise, careful plans to enact revenge on Fortunato for causing him a thousand injuries. The reader also knows that Montresor has a capacity for dissembling and hiding his true emotions, which allows him to get close to Fortunato and avoid suspicion.

In Edgar Allan Poe's "The Cask of Amontillado," the first-person narrator wastes little time in letting his intentions be known. Though not given to the reader right away, we eventually learn that the narrator's name is Montresor. In the first paragraph, Montresor states, "The thousand injuries of Fortunato I had borne as I best could." In addition to these "injuries," we learn that Fortunato has insulted Montresor and that the insult will not be forgiven. Montresor is determined to seek revenge, and he plans to suffer no consequences as a result of his actions.

In the second paragraph, Montresor explains that he has given Fortunato no reason, whether through words or actions, to suspect any wrongdoing or negative intentions. Montresor will continue to, "smile in his face." The reader knows the reason behind the smile is revenge. However, Montresor will not make this reason known to Fortunato.