Montresor said, "To your long life!" to Fortunado, but it was mean another. Montresor planned to kill Fortunado at that time, so it wasn't the same meaning as he said. Montresor knows that Fortunado will die.
For the conflict of this story, in the past, Fortunato insulted Montresor and his family. Because of this event, Montresor planned to kill Fortunado.
In the very first part of the story, Montresor wanted to revenge Fortunado. But on the other hand, Montresor was afraid of being caught. So he agonized over it.