Poe employs this literary device often to induce the reader not only to study Montresor, but also for self-examination. The narrator obviously has something wrong if there are so many arrows pointing for the audience to investigate him. Montresor is taking things a little too seriously and goes a little far in fulfilling his family legacy. Upon entering the catacombs, Fortunato asks the family arms of the tomb, a snake being stepped on taking revenge by biting the heel, and its motto, "Nemo me impune laces sit." The rough translation from Latin means, "No one injures/harms me with impunity." Hence, Montresor is feeling pressure from family duty to uphold the honor of the crest and not allow the so-called "injuries" handed down by Fortunato to go without retribution. The imagery of the snake implies the sleaziness of Montresor's character and references the story of the Garden of Eden where a serpent tempted Adam and Eve to commit evil by luring them to a certain place to consume. The story's end brings light on his further light onto his paranoia and guilt. Just as Montresor is about to finish the wall, Fortunato begins to laugh because he believes the whole incident is merely a practical joke. He never knows why he actually has been laid to rest within the tombs. This troubles Montresor immensely and it is why he is now telling the world the grim facts of his most heinous crime. The mad and criminally insane often boast about their dirty deeds after they have kept it inside so long and the weight of the incident mulls inside them, sinking deeper, until they finally hit that point of no return. They cannot stand it any longer, no one knowing their well-crafted plan, that they tell all. Montresor has hit the point and his mental anguish is apparent the audience examines his state of mental narration.
The irony is slathered onto "Cask of Amontillado" quite heavily in both the verbal and dramatic form.