The Cask of Amontillado" must take into account the story's point of view. Everything we know is filtered through the demoniacal brain of Montresor. Montresor is easily offended, jealous of Fortunato, and a little strange. His propensity for being offended stems from his insecurity, an insecurity that could lead him to invent a story about masterminding the perfect murder/revenge. His jealousy of Fortunato leads him to slant everything in the story to make Fortunato look stupid--his motley dress, his drunkenness, his pomposity. The story itself indicates Montresor lacks sanity and cannot be trusted. It's his insanity, however, that leads the reader to believe he is capable of such an act.