Poe gives Montresor wonderfully villainous characteristics in "The Cask of Amontillado."
Montresor's murder of Fortunato is deviously premeditated. Montresor knows how he will lure him underground and how he will carry out the murder. He has everything in place to entomb the man before he even finds him, and he uses Fortunato's weakness for wine to guarantee that the man will agree to accompany him.
Other characteristics of Montresor's villainy include his pretense of concern over Fortunato's health. Repeatedly he asks after the other man's cough, and suggests they return above. The more he insists, however, the more Fortunato wants to continue on, which Montresor, no doubt, relies on.
Montresor also draws the other man's attention to the damp of the catacombs, knowing that soon he will entomb the sick man in the damp "cave" he has created for him.
Perhaps the most villainous characteristic of Montresor is his seeming madness. Although he never gives the reader a reason, and we can find nothing that suggests Fortunato deserves such treatment, Montresor is committed to taking the other man's life for what seems to be an "imagined" insult.
Montresor has many villainous characteristics which add to macabre mood Poe creates, yet again, in this short story.