Montresor waited for the right moment to get his revenge. He waited until Fortunato was drunk to trust him without question
montresor was fed up being humiliated by fortunato so he made it his mission to get revenge of montresor
"The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge."
Foutonato was extremely drunk when Montresor persuade him to go try the amontillado in the catacombs.Due to his ego and pride he didn't let anyone try the amontillado other than himself which got him killed the day before Ash Wednesday.
“For the love of God , Montresor"
due to furtunato's pride, he let his overconfidence and drunkenness take over his conscious and he trusted montresor without a doubt.that led him to his death.