What do you learn about Montressor from his anaylsis of revenge and from his family's coat of arms in "The Cask of Amontillado"?

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.

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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 

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“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
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"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.