Can you think of any other stories, books, or movies that convey a similar theme as in "The Cask in Amontillado"?
The theme of revenge is also explored in A Tale of Two Cities. In the book, Madame Defarge wants revenge for what the nobles did to her sister. She knits the names of her victims as she waits for the guillotine. She feels no remorse, even to kill the young daughter of the son of a noble when both have done nothing.
"The Cask of Amontillado" by Edgar Allen Poe focuses on a main character who lets his obsession drive him crazy. At least two other Poe stories have this same trend "The Black Cat" and "The Tell-Tale Heart." Come to think of it "William Wilson" had an obsession, and while it did not drive him crazy, it did lead to his death.
For more casual reading, I think of books by Thomas Harris: Silence of the Lambs, Hannibal, and Hannibal Rising, which all deal with the psychotic Hannibal Lecter. In these books the narrator is an objective observer but Dr. Lecter is clearly crazier than a bedbug.
Also, for a really unreliable narrator, who is not sociopathic but definitely lives in his own separate reality, try Shutter Island by Dennis Lehane (read the book, ignore the movie).
A better book, in which each of the narrators is unreliable for his own reasons, is The Sound and the Fury, by William Faulkner.
In Shakespeare's Othello, Iago twists the truth in order to destroy Othello. Iago does not appear insane, just evil.
Akiro Kurosawa's classic film Rashomon shows how every observer of an event interprets that event differently. The idea that "reality" is different for every person has started many discussions on the nature of "reality".
You could compare "Cask" to Alexandre Dumas's Count of Monte Cristo. Both works focus on the protagonist's seeking revenge. While Poe's "Cask" does not make it clear if Montresor has reason to want revenge upon Fortunato, Dumas's Count gives good reason for Dantes' desiring revenge against all who led to his unjust imprisonment. The works give an interesting perspective on two elaborate schemes to get revenge.
I think one of the key elements of this excellent and spine-chilling short story is the fact that Montresor as a narrator is completely unreliable and clearly psychotic. This is a tactic that is used by Poe in a great deal of his short stories - the unreliable narrator is an object of fascination for so many films which fit into the genre of horror. For example, you might like to consider Sixth Sense, when the main character himself only realises a central truth that has happened to him at the very end of the film. Likewise you might want to look at such stories as The Turn of the Screw, which is an excellent ghost story from Henry James. Crucial to its success is an ambiguity about the limited narrator who is most definitely unreliable - the central crux lies around whether the ghosts she imagines to be wanting to possess her wards are real, or just a figment of her fervent imagination. The last scene raises this question very well as we are forced to ponder if she has suffocated the tender Miles or if his heart has been stopped by terror. Excellent.windbag
I strongly advice Hannibal by Thomas Harris, I Have No Mouth And I Must Scream by Harlan Ellision
One of the subjects covered in the short story is revenge. I would think just about any modern criminal film or novel will also have a similar topic and can give you great themes about how people struggle with the world around them and the place of evil in the world.