The snake symbolizes Fortunato attacking or insulting Montresor then Montresor crushes the snake or Fortunato with his foot (Shmoop).
In The Cask of Amontillado Montresor tries to convince Fortunato to go back upstairs by saying “You are rich, respected, admired, beloved; you are happy, as once I was (Shmoop).” Poe uses diction in many ways to convey his point to readers or to emphasize point by repeating the same word more than once. In The Cask of Amontillado Poe uses repetitive words to emphasize disgust, "Ugh! ugh! ugh! --ugh! ugh! ugh! --ugh! ugh! ugh! --ugh! ugh! ugh! --ugh! ugh! ugh! (Edgar Allan Poe).”
The Cask of Amontillado but the cask has a twist of betrayal to the theme as well (Spark Notes). This is critical to the story because Fortunato betrayed Montresor and Montresor vowed revenge but in the end Montresor betrays Fortunato. Although Poe never tells us exactly what Fortunato did to betray him “THE thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge. (Shmoop