In “The Cask of Amontillado," Edgar Allen Poe’s powerful tale of revenge, the reader sees how an obsession with vengeance can overwhelm a person.
When we meet the narrator, Montresor, he is vowing revenge upon Fortunato.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge.
He tells readers he’s been wronged, but he never reveals the nature of the injury. All we know is that Montresor has been preoccupied with these wrongs. But what Fortunato has actually done is immaterial; Poe’s point is how revenge can drive a person to extremes. Montresor declares:
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.
With this statement, we see how powerful Montresor’s need for revenge is. He seems to understand that revenge is a force powerful enough to “overtake” the party who seeks it, but he is too fixated on his need to recognize that his mind has become a slave to this passion.
Because he never actually reveals the actual insult, Poe illustrates that the narrator’s vindictiveness is not really connected to Fortunato’s crime. It is more closely linked to Montresor’s family legacy of vengeance. His family motto is "Nemo me impune lacessit,” or "No one assails me with impunity." The coat of arms bears a
... huge human foot d'or, in a field of azure; the foot crushes a serpent rampant whose fangs are embedded in the heel.
We can see from this image that Montresor has been reared by a family who put revenge at the center of their values, judging by how they chose to represent themselves on their coat of arms. Therefore, it makes sense that his plan to kill Fortunato is premeditated and well thought-out. He has located the right spot, made sure he has the tools of bricks and mortar available, and even figured out a tortuous and gruesome method to kill him. He will make him suffer the anguish of a slow death by watching Montresor wall him up alive, ignoring Fortunato’s screams. Then he will experience the agony of burning alive. This is a cold-blooded revenge, and fifty years later, the narrator feels no remorse. He ends the story with an icy, ironic, “In pace requiescat!” (Rest in peace!)
Another interesting point is that an integral part of the success of this plan is the way Montresor lures Fortunato to his doom. He appeals to Fortunato’s ego, telling him that he is on his way to find Luchesi, one of Fortunato’s rivals, who can determine if the wine Montresor recently bought is actually Amontillado.
"As you are engaged, I am on my way to Luchesi. If any one has a critical turn, it is he. He will tell me—"
"Luchesi cannot tell Amontillado from Sherry."
"And yet some fools will have it that his taste is a match for your own."
"Come, let us go."
The mention of his rival triggers Fortunato’s massive ego. Montresor, feigning concern over Fortunato’s cough and obvious illness, actually gives him several chances to turn back. But Fortunato insists on going forward because he cannot stand the thought that Luchresi could be on a par with him. His pride leads to his demise, which could be considered a secondary theme of the story.