Fortunado had somehow gone too far with his insults towards Montressor, and it finally drove Montressor over the edge. In the beginning paragraph Montressor says that he would get his revenge. "At length, I would be avenged; this was a point definitively settled."
The reader must take the word of Montressor because we never read anything that Fortunado specifically says or does that would constitute "injury" or "insult." The interaction they have begins on the streets during a carnival. Montressor finds Fortunado completely drunk and lures him into his family's catacombs. Previously, Montressor gets rid of his servants for the night, knowing his plan would require him to be alone in the house. Fortunado is very willing to go check out Montressor's cask of Amontillado. Montressor even uses reverse psychology to get him to commit and go with him.
"My friend, no; I will not impose upon your good nature. I perceive you have an engagement--Luchesi...My friend, no. It is not the engagement, but the severe cold with which I perceive you are afflicted . The vaults are insufferably damp. They are encrusted with nitre."
The rest of the interaction between the two occurs on their journey deeper into the catacombs where he buries him alive. Most of that interaction involves his use of verbal irony.
Montressor, the narrator, makes a vow at the beginning of the story that he will get his revenge on Montressor. Throughout the story, he works toward this goal until it is finally achieved and Fortunado is dead. Montressor does not like Fortunado for the "thousand injuries" that he has caused the narrator. Montressor lures Fortunado into the catacombs beneath his family estate by promising him a taste of some Amontillado, very rare wine. Fortunado, who prises himself on his expertise in wines, is easily lured by Montressor's trap. While they are in the catacombs making their way toward this wine, Montressor tries to gain Fortunado's trust by pretending that he cares about him. He also tries to make Fortunado feel that if he is not up for this Amontillado challenge, he can easily be replaced by a man named Luchesi. Eventually, Montressor leads Fortunado to the end of the catacombs, builds a brick wall in front of him, and burns him alive -- needless to say, Montressor gets his revenge and kills Fortunado and prides himself on the fact that no one has found out about this in 50 years.