We don't know specifically what transgressions Fortunato committed against Montresor to cause such an urge for revenge. We only find that
THE THOUSAND INJURIES of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
There are no other specific details to the "injuries" Fortunato had committed. As for the steps that Montresor took toward committing his perfect crime:
- Montresor gave no clue that he was planning retribution, continuing to "smile in his face."
- Montresor conceived the story of the rare bottle of Amontillado, knowing that Fortunato would not be able to resist the temptation of tasting such a vintage.
- Montresor waited until "the supreme madness of the carnival season" to enact his plot. By doing so, he made sure all of his servants would be absent from his home. He also knew Fortunato would be drinking heavily--dulling his senses and reactions.
- He planned for the murder to be undertaken at night, providing less visibility and chance for other people to be around.
- Montresor concocts the story that a friend, Luchesi, will be available to sample the Amontillado should Fortunato decline, knowing full well that Fortunato will not let this opportunity escape him.
- Montresor chooses a safe place--his family catacombs--to complete the crime and dispose of the body. In doing so, he assures that there will be no one to disturb them or hear Fortunato's cries.
- Montresor prepares the "niche" in advance, placing bricks and mortar hidden nearby; he also brings a trowel, the proper tool to cement the bricks. He has a chain ready to "fetter" Fortunato in place. The niche is in a distant area of the vast catacombs, making it less likely that anyone will ever disturb the area.
- Montresor never reveals his crime to anyone--at least for "half of a century."