Why does Dickens change feelings toward the peasants throughout the novel?

Ruth Glancy, Canadian professor and  authority on
A Tale of Two Cities, contends that Charles Dickens takes a
Romantic attitude toward history.  Influenced by his reading of Thomas Carlyle's
History of the French Revolution, Dickens romantically personified
the causes of the revolution as Hunger and the Farmer and the Mender of Roads; he
perceives the conditions of hunger and oppression as the main causes for the revolution
of the peasants against the aristocracy of France. Towards these oppressed peasants,
Dickens is sympathetic.  However, once the revolution begins, the bloodlust and desire
for revenge, personified as the Vengeance, become obssessive and
excessive. 

This contrast between the peasants as they are
earlier in the novel is exemplfied by such characters as Ernest Defarge and even
Gaspard.  M. Defarge, former servant of Dr. Manette, sympathizes with the physician's
imprisonment in the Bastille and is yet loyal to him.  He pleads with his wife to spare
the lives of Charles Darnay, ne Evremonde, and his family.  After the spy John Basard
departs their shop, Defarge says to his wife,

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"But it is very strange...that, after all our
sympathy for Monsieur her [Lucie's] father and herself, her husband's name should be
proscribed under your hand at this moment, by the side of that infernal dog's who has
just left us?"

However,
Madame Defarge is bloodthirsty in her desire for revenge against the Evremonde's, for
she wants the entire family eradicated:

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"Stranger things than that will happen when it
does come...I have them both here, of a certainty; and they are both her for their
merits; that is enough."

Also
in contrast to Madame Defarge is the father whose child is run over by the carriage of
the Marquis d'Evremonde, Gaspard, who seeks revenge, but his is only against the
murderer, the Marquis, and not the entire family.  But Madame Defarge becomes maniacal
in her lust for blood and revenge, rushing to kill Lucie before her husband can save her
and her child.  Like the Vengeance, Madame Defarge desires the death of all aristocrats
and anyone connected to them, such as the poor seamstress who innocently goes to her
death. 

Thus, it may be that while Dickens sympathizes with
the peasants who revolt against oppression, he does not approve of those who
indiscrimately slaughter anyone that they decide is their enemy, acting much as those
who dominated during the Reign of Terror.