How does Poe use writing strategies such as literary elements, techniques, or rhetorical devices to develop the central idea of "The Cask of Amontillado" - that is, a justified revenge?
One writing strategy which Poe uses to develop the central theme of justified revenge is that of imagery. The entire story is peppered with images and motifs which cast Montresor as a wronged avenger, and Fortunato as a simpering, dim-witted fool who is deserving of death. That these elements are so intricately and plentifully woven into the story is testament to Poe's genius. A particularly striking example is that of Montresor's coat of arms, in which a foot is depicted as having crushed a serpent. The serpent, however, has already dug its fangs into the skin of the foot. The coat's accompanying motto, written in Latin, is translated as "No one attacks me with impunity." The message is clear: Montresor envisions himself as a wily serpent who attacks, and poisons, the one who has oppressed him.
Further examples of imagery include Fortunato's costume, which is that of a clown or jester. Its bright colors and clanging bells contrast sharply with the stark and brooding figure of Montresor. Again, Montresor is the avenging angel, and Fortunato is the foolish foot. The entire setting of the story - a dark, decaying catacomb - also functions as imagery, in that Montresor leads the unwitting Fortunato to a terrfying and tomb-like place, where Fortunato will be (literally) buried. Silly Fortunato is drawn into a symbolic and actual tomb by the cunning Montresor, as punishment for his many sins.
While imagery is a crucial part of Poe's depiction of a justified revenge, one can argue that perspective is just as powerful, if not more so. Indeed, it is Poe's use of perspective - that is, his authorial choice to tell the story from Montresor's viewpoint - which creates and nurtures a sense of righteousness in vengeance. Because Montresor is our sole narrator, we know only what he hells us; we must believe him when he proclaims that Fortunato has insulted and debased him, and that he deserves to die. For all we know, Montresor is lying, and Fortunato is totally innocent. We can only imagine what the story would be like if Fortunato were to narrate.
Montresor believes that his revenge is justified; therefore, within the context of his story, narrated by him, it is so. Also, his narration allows for expressions of his hatred to roam freely throughout the story. He constantly mocks Fortunato, claiming that he will not die of a cough, and ends the story with an acerbic, sociopathic jab ("In pace requiescat! [May he rest in peace!]"). Montresor is keen to let us, his audience, feel his righteous anger, and his pleasure in finally having punished Fortunato. These remarks, alongside perspective and imagery, help create the justified revenge of "The Cast of Amontillado."