Quote 1
"To be registered, as doomed to destruction," returned Defarge.
[…] "The chateau, and all the race?" inquired the first.
"The chateau and all the race," returned Defarge. "Extermination." (2.15.63-67)
"Extermination" becomes a pretty sweeping term. It’s easy enough to justify violence against the occupants of a chateau… but the extermination of a race?
That’s hard to justify at all—as Dickens makes clear.
Quote 2
"It does not take a long time to strike a man with Lightning," said Defarge.
"How long," demanded madame, composedly, "does it take to make and store the lightning? Tell me." (2.16.25-26)
Madame Defarge’s implacable rage allows her to wait for the oncoming of revolution. Note the naturalized metaphors in this scene: her rage is as certain (and as natural) as the elements themselves.
Quote 3
"Well, well," reasoned Defarge, "but one must stop somewhere. After all, the question is still where?"
"At extermination," said madame. (3.12.17-18)
If Madame Defarge had her way, there would be no real end to the violence of the revolution. As we see, "extermination" becomes an all-consuming and vastly unjust undertaking.