You could begin by discussing the literary device that Montresor uses to emphasize his feelings as well as his high level of diction and its implications. Montresor begins with a hyperbole (or overstatement), that he had borne the "thousand injuries of Fortunato" as well as possible. It is unlikely that Fortunato actually wounded him so often, but Montresor feels as though he has. It is almost a way to begin to justify what he's done, by insisting that he had patiently endured injuries time after time until, finally, he could bear it no more.
Further, his very high level of diction indicates his education and intelligence, and his cleverness is certainly needed in order to plan and execute a crime for which he can never be suspected. He feels that he must punish Montresor with "impunity," meaning that he will not be punished for it. He desires Montresor's "immolation," his total destruction. We likely have less trouble believing that this person could get away with such a crime because of his intelligence; in fact, it might even be that his intelligence is the source of his pride and thus the reason that he feels he must take down his foe. Moreover, it might even be his intelligence, indicated by his diction, that would lead him to believe that he would feel no grief for what he deems to be the much-deserved revenge he exacts on Fortunato.