What are some character flaws that bring about Fortunato's own demise?
Fortunato must be a heavy drinker. This is a character flaw in any person and usually leads to serious troubles. The reader must assume that Fortunato is not drunk just because it is carnival season. He shows too much fondness for wine to be moderate for most of the year and then to go wild for just a few days. He is undoubtedly, as Montresor says, a connoisseur of wines, and no one becomes a connoisseur of wines without drinking a lot of them.
Fortunato considers himself a funny man. That is why he chooses to wear a jester's costume during the carnival. A jester in medieval times was a man who was privileged to play cruel jokes on people in order to amuse his master and the courtiers. But a jester could make a lot of enemies. Many of the "thousand injuries" Montresor mentions in the first sentence of his story must have been jokes or jibes.
Fortunato is an egotist. If he were not such an egotist he would not be taken in by his enemy Montresor, who has been flattering him and calling him "My friend" and ""My good friend" for years. Poe gives an example of Montresor's outrageous flattery when the two men are down in the cavern:
"...your health is precious. You are rich, respected, admired, beloved; you are happy as once I was. You are a man to be missed...."
Fortunato is avarious. When he hears about the Amontillado he is anxious to taste it. Montresor says he got a bargain on the wine. Fortunato would like to buy some himself--but he doesn't mention this motive for going to the vaults because Montresor is probably planning to buy more casks for resale if he is assured that it is genuine Amontillado. In that case the two men would be competing and would be bidding the price up. Fortunato is not accompanying Montresor to his palazzo just to accommodate him; he is doing it to taste the wine and to keep Montresor from going to Luchesi, who would be another competitor.