How does the description of the Montresor coat of arms and motto in paragraphs 46–49 contribute to the reader's understanding of Montresor’s character?

Montresor describes his family's coat of arms as follows: "A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heal."
There is a dramatic irony here. The reader knows that Montresor plans to murder Fortunato, and thus we can infer that, from Montresor's perspective, the huge golden foot in the coat of arms symbolizes Montresor himself, and the snake with its fangs "imbedded" in the heel of the foot symbolizes Fortunato. Snakes, ever since the biblical story of the Garden of Eden, have been associated with treachery, and Montresor is convinced that Fortunato is guilty of some kind of treachery toward him. The foot crushing the snake in the coat of arms foreshadows Montresor's murder of Fortunato.
The fact that Montresor describes this coat of arms to Fortunato moments before he kills him implies that he relishes the idea that he knows something that Fortunato does not. Montresor enjoys having power over Fortunato—both in knowing this and in taking his life.
The family coat of arms also suggests that Montresor is a very proud man. The fact that the foot is golden ("d'or") implies that his family has a rich heritage, and Montresor convinces himself that he is acting to honor that heritage by taking revenge upon someone who has supposedly acted to undermine it.
The family motto, "Nemo me impune lacessit," translates to, "No one provokes me with impunity." This emphasizes the idea that Montresor is a very proud man. His pride has been injured by something that Fortunato has done, or at least by something that he thinks Fortunato has done, and this seems to be his sole motivation for murdering him.