During the carnival in Italy, Montresor runs into Fortunato, and offers to share a nice bottle of wine. However, this bottle is in his catacomb. it is late at night and Fortunato seems ill.
Fortunato was making fun on montresor. Montresor didn't like the way he was making fun on him. Montresor wants to get revenge on him.
Montresor was upset that fortunato to montresor have to get revenge. as he said step on the snake the snake is gonna bite. That meaning if fortunato makes fun of him montroser and montriser will get payback