"The Cask of Amontillado" by Edgar Allan Poe was Poe's last short story. It is certainly a masterpiece of horror. The grotesque settiing and madness of the main characters lead the reader into revenge, murder, trickery, and betrayal.
The narrator of the story is the main character or unsympathetic, protagonist Montresor. He is not the normal, heroic main character. The force that drives Montresor is his need to punish Fortunato with impunity. In translation, Poe meant that Montresor wants to commit the perfect crime; and that is exactly what he does. The reader learns at the end of the story that Montresor has told the story as a flashback fifty years later.
Back in the time of this story, when someone insulted another person, often the problem would be resolved with a duel using some kind of swords. This is is not how Montresor wishes to acquire his vengenance.
Why does Montresor seek revenge? Fortunato has injured him not physically but verbally or spiritually many times. The thing that sends Montresor over the edge was an insult that Fortunate inflicted on him. Most people do not kill because they have been insulted:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. You, who so well know the nature of my soul, will not suppose, however, that I gave utterance to a threat.
Apparently, to Poe, the details of the insults or injuries were not important. It was how the insult was repaid that became the crux of the story. This is taking the old cliche in reverse to the fartherest limit: Sticks and stones may break my bones, but words will never hurt me. In this story, this is not the case.
Poe allows some insight into the thinking of Montresor when he gives the family motto and coat of arms. Apparently, if something is done to this family, there will be revenge. The snake being stepped on and turning back and biting the foot indicates there will be vindication.
The story circulates around the trickery, betrayal, and trust held between these two characters. Fortunato trusts Montresor enough to follow him down into the catacombs to taste wine. Then, Montresor feels betrayal by a supposed friend who has insulted that friendship; therefore, he will use a ruse to get his arrogant, drunken "friend" to follow him all the way to his death.