One night Montresor was having a little party with people over. When Fortunato decides to drunkenly insult Montresor, it becomes a completely different altercation. Montresor's family code of arms essentially says if you disrespect me, I will kill you so after Fortunado says some unkind words.
Montresor thinks of a plan and sends out to find Fortunado, when he does he talks about finding a fellow member of the community who is a wine expert. In hearing this Fortunado bursts out saying that he knows nothing about wine and that he will help with the wine while coughing and wheezing.
After not letting up Montresor agrees to letting Fortunado help when secretly that's what he wanted in he first place.
Montresor keeps telling Fortunado that he will find someone who isn't getting sick to help but Fortunado is clearly getting worse.
Coming upon the end of his wine cellar and in a drunken and sick mess Fortunado is unable to realize what is happening when Montresor stabs him and chains him up to the wall.
Montresor is still talking to Fortunado although building a brick wall trapping him and killing him and leaving him there forever.