Montressor's revenge upon Fortunato in "The Cask of Amontillado" is two-fold: It must be fitting and severe, and it must be completed without the possibility of prosecution or reprisal.
At length I would be avenged; this was a point definitely, settled—but the very definitiveness with which it was resolved precluded the idea of risk. I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong.
Montressor's revenge must be swift, decisive and free of risk, and in these actions he was successful: The completion of his act proved to be a perfect crime. Montressor may have based this goal on his own family motto. The family crest displayed
“A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel.”
The motto read
"Nemo me impune lacessit" ("No one attacks me with impunity").
So, Montressor's crest was emblazoned with a human foot crushing a snake, whose fangs were attached to the heel. The motto simply means that no personal attack will go without punishment. Certainly, Montressor followed both of these ideas when he decided to kill Fortunato. Like the snake, Fortunato had somehow insulted or attacked Montressor; like the heel, Montressor would respond by crushing his enemy.