Is the narrator of Edgar Allan Poe's "Cask of Amontillado" also a murderer?

In Poe's short story "Cask of Amontillado" the narrator, Montresor, makes his intentions clear in the first paragraph. He has been insulted by Fortunato and views the man with complete disdain. Because of this unnamed insult Montresor vows to punish Fortunato, but makes it clear that any punishment must go undiscovered and that Fortunato should be the only one to know that Montresor has punished him. The narrator says,

"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."

Montresor has devised a plan for murder which will not only go undetected by any outside entity but as the murder unfolds, Fortunato will be quite aware of who is responsible for his demise. Montresor chooses the perfect setting for his crime, the Carnival season. Carnival (literally a farewell to meat), as its name suggests, was a time of excess and extreme (picture the annual Mardi Gras celebration in New Orleans) in anticipation of the Christian observance of Lent, a forty day period of fasting and abstinence prior to Easter. Costumes and alcohol are in abundance during this festival. 

Not only is Fortunato inebriated but he is also dressed in costume as he comes under Montresor's plot of retribution. Montresor lures the drunken man to his villa with the excuse of needing his opinion on a rare spirit called Amontillado. Because of the festivities Montresor's servants are all out of the house enjoying themselves.

As was the custom of the wealthy in those days Montresor keeps his wine collection underground in what were once catacombs, or underground cemeteries, which is, of course, a fitting place to perpetrate his crime. As he and Fortunato descend deeper into the vault Montresor continually praises his victim about his knowledge of good wine to make it look as though this is simply two friends indulging in a common interest. Montresor is also plying Fortunato with more drink so that by the time Fortunato realizes what is happening it will be too late.

Finally toward the bottom of the incredibly creepy vault Montresor chains Fortunato to the wall and proceeds to actually wall him in, thus gaining retribution for the unnamed insult without a trace of evidence. Fortunato will eventually die in the catacomb and his bones will mix with the others buried in this underground tomb. The narrator concludes, "For the half of a century no mortal has disturbed them." As Montresor predicted his plan works perfectly and only Fortunato realizes the true identity of his murderer.