Is the murder in Poe's "The Cask of Amontillado" the direct result of Montresor's pride, Fortunato's pride, or some combination of the two?
I believe that the murder in Edgar Allan Poe's short story, "The Cask of Amontillado" is a combination of the pride of both men.
We quickly suspect that Montresor is insane. He has decided he must kill Fortunato for some unnamed reason. And we never find out, even by the end of the story, what insult (real or imagined) Fortunato has paid Montresor.
Montresor makes it very clear in the story that his pride is at the root of the perceived insult, as well as his motivation to murder Fortunato.
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.
Our understanding of Fortunato's pride comes first from an observation by Montresor.
He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseurship in wine.
Montresor, knowing the personal pride (and enormous ego) Fortunato has regarding his knowledge of wines, uses the imaginary "cask of Amontillado" to lure Fortunato into the catacombs. His offer to call upon another "expert" (Luchesi) because of Fortunato's limited free time and his cold only whets Fortunato's appetite all the more to see Montresor' treasure.
“My friend, no; I will not impose upon your good nature. I perceive you have an engagement. Luchesi—”
“I have no engagement;—come.”
“My friend, no. It is not the engagement, but the severe cold with which I perceive you are afflicted. The vaults are insufferably damp. They are encrusted with nitre.”
“Let us go, nevertheless. The cold is merely nothing...
Montresor's accurate appraisal of Fortunato's pride makes it all the easier to guarantee that the other man will insist upon accompanying him underground where the imaginary wine is supposed to be stored. It is also the season of Carnival, and as Fortunato has been drinking already, Montresor has no difficulty in getting the other man even more inebriated as they walk so that Fortunato suspects nothing.
Montresor's injured pride—whatever the cause—remains his focus for the entire story. When Montresor's family crest and motto are discussed by the two men, it is easy to see that Montresor's pride is something he was "fed" as he was growing up. His family's motto is: “Nemo me impune lacessit” which means "no one attacks me with impunity." With this piece of information, we can well understand that as long as Montresor imagines his pride has been injured, he will not forgive and forget.
In that Fortunato is so easy to manipulate by playing to his ego indicates what an overblown pride he has. That, and his drinking, make it impossible for him to think clearly, and he is unable to protect himself.
Montresor is as predatory as a fox in the company of chickens: Fortunato is lost as soon as he agrees to go with Montresor. Because each man has an enormous pride, the murder assuages Montresor's injured pride, and Fortunato's pride leads him to his doom.