The storming of the Bastille happens in "Book the Second: The Golden Thread - Chapter 21: Echoing Footsteps" in Charles Dickens's A Tale of Two Cities.
The takeover of the Bastille is fierce, but the Defarges both fight valiantly. When he is able, Ernest Defarge charges into the Bastille and immediately goes to a particular cell, 105 North Tower, where he searches the cell.
When he is finished, Defarge returns to lead the mob, and Madame Defarge follows the "grim old officer" like a stalker, staying "immovable close to him." She stays close to him until he gets close to his destination and everyone in the mob begins to beat and stab him. She did not participate in this violence, just staying near until he fell down, dead.
[S]uddenly animated, she put her foot upon his neck, and with her cruel knife—long ready—hewed off his head.
Madame Defarge is the one who decapitates the Bastille guard.