The mood at the end of the story is disturbing and suspenseful, because the reader realizes that Fortunado is going to die.
Mood is the emotional landscape of a story. It is created by the author’s tone, word choice, and other elements including setting and plot.
Montresor seems to think that his friend Fortunado has wronged him in some way. He does not indicate why, but he is planning an elaborate scheme to get revenge. Fortunately for him, the carnival is in full swing and there is a lot of distraction and chaos.
Suspense builds as Montresor leads Fortunado deeper and deeper into the catacombs in search of an old cask of wine. What they reach is not wine, but a stockpile of bricks and mortar. Since Fortunado is drunk, he is not sure what is going on until Montresor starts walling him in. At first, he thinks it’s a joke
“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.”
At this point, the reader is beginning to feel more and more worried. Montresor is about to kill Fortunado, and it seems like he is going to get away with murder. He has no guilt, and actually seems to enjoy the process, gleefully teasing his victim. This creates the mood that is both disturbing, because a murder is taking place, and suspenseful, because we do not know what is going to happen.