Consider how the family motto and the family crest relate to Montresor's anger. Consider also Fortunato's reactions when Montresor tells him about the crest.
Montresor's family motto, "Nemo me impune lacessit," or, you will not harm me with impunity, means that his family is most known or most desirous to be remembered for the fact that they will never allow someone else to hurt them without exacting some form of retribution. Therefore, their family crest shows a snake being stepped on by a human foot, but that snake is simultaneously curled around to bite the foot that steps on it. It is possible, then, that Montresor's family actually relates to the snake, as if to say, "You might hurt me, but I will make you feel pain as well." In other words, one cannot get away with harming a member of this family without feeling some painful consequence.
This is exactly what happens to Fortunato. Whether it is true or not, Montresor feels that he has borne a "thousand injuries" inflicted by Fortunato; thus he vows that "[he] must not only punish, but punish with impunity." He feels that he has been incredibly wronged, and now he will bite back, so to speak. Further, it is of utmost importance to him that he exact his revenge without being caught or having to pay a price for it. Just as his family will not allow themselves to be harmed without some form of retribution, Montresor feels that this retribution will not be complete if it cannot be wrought without consequence.
The Montresor family crest (or "arms") is "A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel." That is (in plain English), a huge golden human foot on a sky-blue background; the foot is crushing a snake with the head in profile (the archaic definition of "rampant"), and the lizard's fangs are sunk into the heel.
The implication of this imagery is that Montresor's family may occasionally get "bitten," but they win in the end (by crushing their enemies).
Fortunato doesn't put this together, possibly because he is drunk. He says, "And the motto?"
Montresor replies: "Nemo me impune lacessit." In English, that is, "No one cuts me with impunity." (Or, in clear modern English, "No one hurts me and gets away with it").
Fortunato merely says: "Good!" He is drunk (as mentioned), but also completely oblivious to what Montresor has in mind. Considering the fact that Montresor is clearly a bit mad, it's entirely possible that Fortunato hasn't even done anything truly bad to him; if this is the case, he has not reason to believe Montresor plans to harm him.