It is pride that drives Montressor to vow revenge just because of an insult. As stated in the lines " The thousand injuries of Fortunato I had borne as best as I could but when he ventured upon insult, I vowed revenge", we could noticed this pride in the personality of Montressor. He doesn’t want that anyone could put him down because he would appear to be weak and superior, so he plans his revenge carefully. Montressor's description of the coat of arms of his family wherein the shield features a human foot crushing a serpent with a family motto " Nemo me impune lacessit,' which means “No one may insult me with impunity" relates with it his great desire for revenge. That is the reason why his decision or plan for killing Fortunato is firm. He never changes his mind from the moment they entered in the catacombs until he had murdered Fortunato. Montressor's pride and beliefs really lead him to be immoral.
Fortunato died because of his pride on his connoisseurship in wine. Montressor knows the weak point of Fortunato, that is, his connoisseurship in wine. Montressor uses the Amontillado as bait so that his plan of revenge will be successful. He even teases Fortunato that he had a plan of letting Luchesi to taste the wine to insure that