Montresor, like many of Poe's protagonists, is mentally disturbed, and thus, his narration is unreliable. We are apprised of Montresor's reason for seeking revenge against Fortunato. We do, however, quickly become aware that he is obsessed. Montresor has given his staff the evening off for the festival, so that he can have Fortunato alone. He has also has left masonry equipment in the catacombs. Once he has Montresor alone, he provides him with vague details on how Fortunato has to pay (for his insults) and since he hasn't tried to make things right, Montresor will take matters into his own hands and punish Fortunato himself. Motressor is clearly delusional.