There are several repeating motifs throughout The Case of Amondillado by Poe. One of them is the use of the celebration of Carnival; a time like Mardi Gras or Halloween when people dress up in costumes and enjoy a certain liberation from their usual personas. This allows someone to act in a way that they normally might not.
Secondly, there is the motif of the snake ready to strike. In fact, Montressor's family crest has a snake on it. He likens himself to an animal readying itself for the attack.