We are lead to believe that Montressor is mad because he gives no reason or motive for his revenge. He says that Fortunato had born him a thousand injuries, but he cannot give any specific example that warrants murder. More, Fortunato does not suspect his scheme at all. This suggests that Montressor is more than just a little sensitive, to the point of paranoia and schizophrenia (detachment from reality).
Also, Montressor is able to remember the entire story as if were yesterday, when--in fact--it has been over 50 years. This suggests that Montressor has played and re-played the revenge in his mind, taking satisfaction in every last detail--to the point of megalomania.
One other scenario is that none of this ever happened: Montressor may have invented the entire plot! We cannot be sure. Perhaps this is the most disturbing.
Lastly, Montressor takes joy in suffering and death. He even mentions his work in connection with "the love of God." Look at the end for clues:
“Ha! ha! ha!—he! he! he!—a very good joke, indeed—an excellent jest. We will have many a rich laugh about it at the palazzo—he! he! he!—over our wine—he! he! he!”
“The Amontillado!” I said.
“He! he! he!—he! he! he!—yes, the Amontillado. But is it not getting late? Will not they be awaiting us at the palazzo, the Lady Fortunato and the rest? Let us be gone.”
“Yes,” I said, “let us be gone.”
“For the love of God, Montresor!”
“Yes,” I said, “for the love of God!”
But to these words I hearkened in vain for a reply. I grew impatient. I called aloud—
No answer. I called again—
No answer still. I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells. My heart grew sick; it was the dampness of the catacombs that made it so. I hastened to make an end of my labour. I forced the last stone into its position; I plastered it up. Against the new masonry I re-erected the old rampart of bones. For the half of a century no mortal has disturbed them. In pace requiescat!