Answer :
Montressor is angry with fortunato because he feels that fortunato has insulted him. Earlier in the story montressor says "The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge." We are never really shown what actually made Montessori think this but observing montressor himself he is easily insulted and narcissistic in a way. Montressor wants to keep his plan as secret as possible so he decides to use fortunato s best wine against him. Montressor tricks fortunato into thinking that he has something that could pass for the amontillado. Fortunato quickly gets excited and interested which is perfect for montressor s plan. Fortunato insist that he must try the amantilldo so they go to the montressor s vault where montressor kills fortunato at last.