The mood is the emotional state of the author or narrator in a story.
Montressor's mood in "The Cask of Amontillado" is a combination of the following: vindictive, calculating, sadistic, enraged, manipulative, deceitful, ironic, mad, maniacal, and very paranoid. After all, Montressor is carrying out a premeditated morbid revenge against his enemy without his enemy suspecting him.
Montressor is a Janus character, a two-faced schemer who baits his "friend" into his catacombs only to bury him alive. Here's an example of his ominous tone:
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult 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 was a point definitely, settled—but the very definitiveness with which it was resolved precluded the idea of risk. I 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.
Not only is he honest with us, his confessors, but he is proud of his act of revenge. He's been counting all the wrongs done to him and feels justified in murder. As such, he is mad, maniacal, and very paranoid. Fortunato likely did nothing to deserve his fate, which makes us wonder if Montressor is indeed detached from reality.