Edgar Allan Poe, author of "The Cask of Amontillado," often instilled in his protagonists a sense of morbid ("sick") humor. This is certainly true of Montresor, the main character in this particular story. As the reader progresses follows the plot of the short story, it becomes clear that Montresor must suffer from some degree of mental illness (delusions,etc.).
Montresor, who enjoys his revenge on Fortunato, frequently makes what he considers to be humorous comments to his victim as he leads him toward the site of his imprisonment. This sense of sick humor leads the reader to be truly aware that Montresor is not completely "right." The knowledge that Montresor is mentally unbalanced and that he is eager to repay Fortunato for his wrongdoings cause the reader to feel a sense of suspense. According to Montresor himself, he intends to seek serious retribution for unpardonable acts committed by Fortunato.
...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.
The intensity of Montresor's plans and feelings, coupled with the grotesque humor marking his mental imbalance, lead the reader to understand that something terrible is to come; a heightened sense of suspense is the result.