Madame Defarge is the essence of the French Revolution. Her and her husband's wine shop is the center of the revolution. Throughout the novel, she knits a register with the name and description of every person against the revolutionary cause. She is characterized as being brutal, with absolutely no sympathy for Lucie or any other person against Madame Defarge's cause. By the time Dickens gets around to explaining why Madame Defarge is so brutal, we can no longer empathize with her because she has ceased to be human, but machine. Her vendetta against the Evrémondes is personal and shows in her life's work and through her actions.
Madame Defarge also provides the perfect foil for Miss Pross. Miss Pross personifies order and loyalty, as where Madame Defarge represents the violence of the Revolution.
Dickens also writes Madame Defarge's death at the hands of her own gun by Miss Pross represents that violence, revenge, and grudges will lead to your own downfall.