One of the ironies of "The Cask of Amontillado" is that Fortunato contributes to his own end. In what ways does he do so?
Fortunado, if he had been a bit more wary, humble, and sober, might have avoided his dreadful end in the catacombs. The first thing that does him in is pride, and Montresor milks that pride well. Montresor knows that Fortunado
"had a weak point...He prided himself on his connoisseur-ship in wine."
Fortunado takes pride in his expertise regarding his abilities to discern quality draughts. Montresor appeals to that sense of pride by saying he bought a bottle of Amontillado, but fears that he was ripped off, and so he is going to ask another man, Luchesi, to see if the wine is quality or not. Immediately, Fortunado's pride kicks in, and he says Luchesi is a fool, and that the only man that can do the job properly is himself. So, Fortunado's pride lends a hand in getting him down into the catacombs in the first place.
Once down there, his drunkenness keeps him from having his proper sense of awareness about him; his inhibitions are lowered, and he's a bit fuzzy about things. This prohibits his spidey-sense from going off that Montresor is leading him quite far into the tombs, much further than a typical wine collection would be kept. Montresor continues to milk his pride, saying they should turn back and consult Luchesi, and his drunkenness by giving him more and more alcohol. So, Fortunado's vanity and intoxicated state do indeed aid Montresor in his evil scheme to do away with the insulting Fortunado. I hope that those thoughts helped; good luck!