Montresor's unreliability as a narrator is enhanced by the very elaborateness of his revenge plot. It just seems all so excessive, so over the top. If Montresor had enlightened us to the reason for his seeking revenge on Fortunato, then perhaps we might have found him a more reliable narrator. As it is, it just seems scarcely believable that he'd go to all that trouble just to exact revenge for some unspecified wrong. Besides, a man capable of performing such an elaborate revenge plot based on trickery is inherently unreliable, whether he's telling a story or not. So if Montresor's telling the truth then he's unreliable; and if he's not, then he still is.
And bear in mind also that Montresor's telling us all about this some fifty years after the incident's supposed to have occurred. This merely adds to his unreliability as a narrator.
In the opening sentence of the story, there is evidence that the narrator is unreliable in his vague description of why he chose to murder Fortunato....
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