What explanation does Dr.Manette's letter provide for the actions and vengefulness of Madame Defarge?

We wonder, for the better part of A Tale of Two
Cities, why Madame Defarge seems so angry and so intent on punishing Charles
Darnay--and later his entire family.  When the Defarges are part of the storming of the
Bastille, they go immediately to Dr. Manette's former cell (one hundred five north
tower) and conduct a systematic search.  What they found, we discover later, is the
evidence which incriminates Charles and his entire family as part of the hated French
aristocracy.

The letter was written by Dr. Manette while he
was in prison and recounts the story of his imprisonment at the hands of Charles's
father and uncle, Monsieur the Marquis (twin brothers).  The story is
tragic.

The two Evremonde brothers were cruel and sadistic,
representing all the worst of the aristocratic class.  What they wanted they got--and
they wanted a young married woman in their village.  They took her, and in doing so
humiliated her husband to death, killed her brother in a duel, and caused her father to
have a heart attack which killed him. In an unexpected act of charity toward their
victims, the men virtually kidnapped the young Dr. Manette, who did his best to save the
two who were alive when he got there--the young girl and her brother.  He heard enough
of their story before they died to be dangerous to the brothers.  They returned him to
his home, but their spies intercepted a letter written by Dr. Manette explaining to the
authorities what he had witnessed.  These Evremonde brothers could not allow that, so
they used their influence with the Court to have him
imprisoned. 

It's a tragic story, though we know beyond
question that Charles has not carried on this horrible Evremonde legacy.  What makes
this important to Madame Defarge is that it's her family about which this letter was
written.  Her father, brother, brother-in-law, and sister all died at the hands of this
family--which explains her thirst for vengeance against any heirs.  The letter ends with
Dr. Manette's curse of condemnation on the family for all generations, which makes him
the prime witness against his own son-in-law.  Madame Defarge will not be content until
the entire family has been killed, just as they killed her family.  It's probably a good
thing she dies before they all manage to escape with their
lives.