In Poe's The Cask of Amontillado, "nemo me impune lacessit" is the motto of the Montresor family. It is exemplified in their family crest, which depicts a human foot crushing a snake that is simultaneously sinking its fangs into the heel of the foot.
This is an extremely literal depiction of the motto, which means "No one (offends/attacks) me (unpunished/with impunity)". Both snake and foot are attacking, and both are being punished with violence. The question is, which represents Montresor's family, and which represents their enemies? There are quite different connotations, depending; the foot would appear more oppressive crushing the snake, even in retribution for a mortal wound, whereas the snake in its disadvantaged position seems justified in attacking the foot that treads on it. On the other hand, the human foot is easier to sympathize with for most people, while snakes have a more sinister connotation.
One cannot tell which element represents Montresor, but it would have to be whichever did not strike first - the one that is taking revenge. For that is the role Montresor takes on in the story, avenging himself against Fortunato for the latter's antagonism.