The fact that the snake bites the foot shows that Fortunato has hurt his friend. In reality Montresor has gotten his revenge and has also not been caught. The first paragraph contains an additional idea of revenge.
It seems that something like this is quite natural for Fortunato. Trust is an issue in this story. The snake is still able to bite the foot and the poison now is affecting the soul of Montresor for half a century.
Even today, over years after it was published, it is still being read. When Montresor met Fortunato, he smiled continually at Fortunato, who thought he saw a smile of warmth and friendliness, when in reality, the smile was a satanic smile in anticipation of Fortunato's entombment.