Why, the ultimate revenge, of course. At least that is Montressor's intention. He lures Fortunato into the catacombs and walls him up there. Motressor thinks that murder is the revenge he seeks; however, this act alone doesn't fully satisfy Montressor's need for vengeance. When Fortunato stops crying out or begging for mercy, Montressor taunts his foe, trying to get him to scream or beg, but instead he is answered with silence except for the jingling of bells on Fortunato's hat. See the excerpt below:
"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 - on account of the dampness of the catacombs."
Notice how Montressor excuses his rushing in the end by saying the dampness of the catacombs made his heart grow sick. But an astute reader will notice the dash-which makes the excuse of the dampness seem almost an afterthought-as if he is trying to convince himself that it was the dampness, and not his own disappointment in Fortunato's reaction, or lack thereof, that made his heart sick. So, even though Montressor's plan is a success, it fails to completely satisfy him. He does not get the ultimate thrill of hearing his enemy shriek in horror or beg for mercy. That is what Montressor was hoping for. So his victory over Fortunato is less fulfilling than he had hoped. Fortunato steals his glory by not reacting to Montressor's cruelty.