The narrator and main character is a pre-meditated murderer. This is the basis for “The Cask of Amontillado,” by Edgar Allan Poe. Montresor lures Fortunato to his death. The why and how are the essence of the story.
Revenge motivates Montresor and the story. Fortunato and Montresor have an odd relationship. Apparently, Fortunato says things without thinking, and his hubris does not allow him to realize that someone might take offense at what he says.
Montresor appears to be telling his story to someone who has more knowledge of his character than the reader ‘‘You, who so well know the nature of my soul.” Montresor has pledged to take his vengeance because Fortunato has insulted him. He further states that Fortunato has injured him a thousand times but gives no details about those injuries.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
It is the insult that takes him over the edge.
Apparently, Montresor’s family has never taken an affront lightly. Even their family motto indicates that they punish those who attack them and without being punished themselves [impunity]. Further the coat of arms displays a snake being crushed by a foot but the snake gets its revenge by turning and biting the foot in the heel.
'I forget your arms.'
'A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel.'
'And the motto?'
'Nemo me impune lacessit.' [No one wounds me with impunity.]
For Montresor to revenge himself for Fortunato’s insult, he has to get away with it – if Fortunato can revenge pay him back then his efforts will have been useless. The punishment must be permanent − Fortunato has to feel it, and he has to know it is coming from Montresor.
Montresor devises and plans in detail how to seek his revenge from Fortunato by luring him into a place where no one will be able to hear, see, or discover him. To have his victim know that he is going to die and realize that it is because of a ridiculous cask of wine will give great pleasure to Montresor.
.. . I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
Montresor will get his revenge and not be punished. The reader knows that this is true because the entire story is a flashback told by the elderly Montresor fifty years later, and Fortunato's bones have never been disturbed.