Montresor had already had a great anger built up against fortunato because of how many time he had "made fun of him or teased him". Finding out that fortunato had not only said "mean" things in front of him but to find out that fortunato had laughed at his proud name made him only angrier.
Montresor had been holding in his anger every time fortunato had said said something hurtful to his face but when fortunato had said something behind his back montresor snapped. Montresor had begun planning his revenge and in this case that meant death. Montresor then pushed the plan into action. Montresor then reeled fortunato into his trap and bricked him into a cavern while he left fortunato tied to the wall on the other side of this brick wall that montresor had made. That was the end of fortunato.
Some might say that montresor is not a psychopath, that he is just a man with anger issues. I say, would a man with anger issues kill another man just because of a laugh at the name of a family that had mostly died off. Montresor has quite a bit of psychotic tendencies anyway. montresor is very good at manipulating people, being sneaky, and he is charming, grandiose self worth. These are just a few of the psychotic tendencies that Montresor