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.
One must have a little background knowledge of the location and time period in which the short story occurs. This is Catholic Middle Ages Italy. The coat of arms (the snake clasping the heel crushing it) sets the stage for the revenge--which Montressor wants for now and later. Montressor gets Fortunato DRUNK--a mortal damble sin in the Catholic Church unless confessed to a priest. If one dies with this unforgiven sin, it means ETERNAL DAMNATION in hell. This is Montressor's TOTAL REVENGE. Not only does Fortunato realize at the end what is happening, but also realizes where he is going to. And Montressor (the Monster), gets away with total revenge--murder of the body and soul of his enemy Fortunato.
From these two things, we learn that Montressor is a pretty cold-blooded individual who is really quite ruthless when it comes to getting revenge on people who (he thinks) have done him wrong.
We find out that he wants real revenge. He wants the guy to know it's happening to him. He wants the victim to know who is getting revenge.
From the coat of arms, we know that it is his intention to never give up. He will get revenge at all costs.
Overall, then, I guess you could say he's a ruthless and implacable enemy to have.