What does vengeance symbolize in The Cask of Amontillado?

In Edgar Allan Poe's "The Cask of Amontillado, Montresor's so called need for vengeance is his justification for murdering a rival. It also represents the cowardice of the narrator. At this time, the customary reaction to an insult this egregious would have been settled by duel. Yet, Monstresor seeks his vengeance in a very under-handed passive-aggressive way.