Montresor's rather diabolical nature is conveyed as early as the first paragraph of the text when he explains his philosophy regarding revenge. He says that he
must not only punish, but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
He goes on to say that he continues to smile in the face of his enemy, never giving the man cause to doubt his good will or friendship, and Montresor says that Fortunato "did not perceive that [Montresor's] smile now was at the thought of [Fortunato's] immolation." These descriptions help to establish an ominous mood of foreboding, in part, by creating dramatic irony. The reader knows that Montresor plans to destroy Fortunato, but Fortunato himself does not know, and this irony makes for a very tense mood.
In terms of tone, which refers to how the author feels about the subject of the text, it would seem that Poe's tone is rather knowing or matter-of-fact and, perhaps, even a little judgmental. Montresor claims to be speaking to someone who he says "well know[s] the nature of [his] soul" and, later, that it has been "half of a century" since he committed this heinous act of revenge. He evidently believes that he has gotten away with the revenge, that he has incurred no negative consequence as a result of it—just as he specified he must early on in the telling of the story—but one might argue that there are grounds to suggest that he has been punished by his own guilt. First, he could be confessing to a priest on his deathbed, as why else would he tell the story now? If he wishes to be absolved and forgiven, then he must be feeling the weight of guilt on his conscience. Second, Montresor admits that, when Fortunato ceased to speak from behind the wall, his own "heart grew sick," and he "struggled with [the] weight" of the final stone. He chalks up the feeling of sickness to the dampness of the catacombs, but this dampness has not bothered him before, and he has just bricked an entire wall without struggling with a single stone's weight. Poe punishes him with his own conscience, nullifying his revenge and making Montresor just a simple murderer.