Montresor has not been telling Fortunato the truth since he encountered him on the streets and told him he had just bought a cask of Amontillado at a bargain price. Why should the reader believe that the coat of arms Montresor describes to Fortunato is genuine? Why should the reader believe that the motto Montresor describes is genuine? Montresor may enjoy deceiving Fortunato and at the same time hinting that he is going to murder him in revenge for past injuries. Montresor quotes the motto in Latin. Fortunato probably does not even understand Latin. Evidently they have known each other for a long time. The fact that Fortunato inquires about Montresor's coat of arms at this late date suggests that he either holds him in low esteem or else that he knows Montresor has no coat of arms and that in either case this is a subtle insult, a social snub. A comparable insult might be for an Englishman to ask an acquaintance, "Did you go to Eton or to Harrow?" when he knows full well the acquaintance went to some obscure school in a poor part of London.
The reader should not take it for granted that Montresor is seeking revenge in part because he comes from a proud and noble family. He may be filled with hatred just because he does not come from a proud and noble family. The fact that he has a palazzo means nothing. Venice is in decline and few people want the expense of maintaining the crumbling palazzi. Montresor could be renting the place just for show. The bones in the catacombs below the mansion may belong to someone else's ancestors, and he may be prevented from removing them by terms of his lease or by local laws or for some other reason.
The coat of arms gives us a reason to understand why Montressor is so unforgiving in getting revenge from Fortunato. The picture on the coat of arms is one of a golden foot crushing a snake which has its fangs imbedded in the heal of the foot. The motto of the Montressors is "“Nemo me impune lacessit.”. In English this means no one punishes me and gets away with it. Fortunato had evidently insulted Montresor's name at some point. Just as his family coat of arms says, no one will punish or insult him and get away with it. The picture reinforces that image. A human foot is crushing a snake that has just bitten the foot. Evidentally, Montresor considers Fortunato to be the snake, and Montresor is the foot that is soon to crush and kill him.
The coat of arms is an allusion to the bible where it talks about the savior stepping on a serpent and it biting him. This is symbolic for how Montresor feels superior and even like he is the savior. The motto "nemo me impume lacessit." shows why Montrsor thinks that he should revenge on Fortunato, because no one should attack him with impurity.
Much of ms-mcregor said is correct except that Montressor probably views himself as the snake
"“He is an ignoramus,” interrupted my friend, as he stepped unsteadily forward, while I followed immediately at his heels."
Poe purposely uses that imagery so that we imagine Montressor as a snake ready to pounce.