Montresor is not very reliable. He is blunt to the extent that he explains his thirst for revenge and then goes on to explain exactly what occurred when he fulfilled that thirst. However, he is unreliable because he never reveals to the reader what Fortunato has done to him. He only says that Fortunato has, among thousands of previous injuries, insulted him. And Montresor adds that his revenge will not be complete unless he can "punish with impunity." This desire for revenge is the only impulse/motivation that we get. Montresor provides no information about how Fortunato has wronged him and he provides no information about how he feels following the murder and the subsequent fifty years since.
The narrator, Montresor, is reliable to the extent that he explains what happened during the deception and killing of Fortunato, but Montresor is wholly unreliable to the extent that he does not include Fortunato's sin, whether he deserved such a fate, and whether Montresor ever had any guilt about the act. The only suggestion of remorse occurs in the final paragraph:
I thrust a torch through the remaining aperture and let it fall within. There came forth in return only a jingling of the bells. My heart grew sick--on account of the dampness of the catacombs.
The -- suggests a hesitation: Was Montresor about to express some guilt but then lied, blaming his "sickness" on a physical reaction to the dampness? Montresor's unreliability is what makes the story so intriguing. He/Poe leaves the reader wondering about what led up to the revenge and about Montresor's reaction following it.
The reliability of Monstresor's story is questionable, particularly because he never specifies what Fortunado's offense was. Other essayists have also noted that Fortunado's behavior(his eagerness to sample a fine wine while intoxicated or his classification of Amontillado as a wine that is not a sherry) do not suggest that Fortunado was a connoisseur of wines.
"Though Fortunato is presented as a connoisseur of fine wine, Cecil L. Moffitt of Texas Christian University argues that his actions in the story make that assumption questionable. For example, Fortunato comments on another nobleman being unable to distinguish Amontillado from Sherry when Amontillado is in fact a type of Sherry, and treats De Grave, an expensive French wine, with very little regard by drinking it in a single gulp. Moffitt also states that a true wine connoisseur would never sample wine while intoxicated and describes Fortunato as merely an alcoholic. Moffitt also suggests that some people might feel Fortunato deserved to be buried alive for wasting a bottle of fine wine."
Consequently Montresor could be exagerating his crime. He claims to have sucessfully fooled and murdered a man of high social standing without being caught and punished. However, his narration reveals that perhaps Fortunado was not necessarily of this noble status.