Fortunato possesses several significant character flaws that lead to his demise in Poe's classic short story "The Cask of Amontillado." Fortunato's insensitive, confident personality prevents him from recognizing Montresor as a malevolent enemy, who is carefully plotting against him. Montresor mentions in the first paragraph of the short story that Fortunato had caused him a "thousand injuries," which implies that Fortunato is a relatively inconsiderate, rude man.
Fortunato's affinity for wine and his excessive pride are also significant character flaws that lead to his demise. Fortunato is clearly inebriated during his interactions with Montresor, which affects his judgment and causes him to let his guard down. Montresor is also aware that Fortunato takes pride in being a connoisseur of wine, which is why he mentions that he is going to ask Luchresi's advice regarding the rare Amontillado wine. Montresor realizes that Fortunato will object, criticize Luchresi, and insist on identifying if the wine is authentic or not. Overall, Fortunato's character flaws impact his decision-making abilities and prevent him from recognizing Montresor as a dangerous enemy. His excessive pride and affinity for wine also contribute to his terrible decision to follow Montresor deep into his family's catacombs.