How did Ernest Defarge die?

Ernest Defarge is a wine-merchant in a particularly unpleasant slum neighborhood of Paris. He and his wife—the unspeakable Madame Defarge—are both fanatical supporters of the French Revolution, whose main movers were not, as is commonly supposed, the great unwashed, but sturdy members of the prosperous middle-classes like the Defarges.
Though fiercely committed to the revolutionary cause, Ernest nonetheless maintains a shred of mercy and decency, refusing to go along with his wife's diabolical plan to murder sweet Lucie Mannette. This causes Madame Defarge—the French Revolution's answer to Lady Macbeth—to accuse her husband of weakness. It seems that Ernest has no stomach for what his wife thinks needs to be done to advance the cause of the Revolution.
Madame Defarge is subsequently shot dead after her pistol goes off during a struggle with Miss Pross, Lucie's governess and friend. As to her husband, his fate is unknown, but Sydney Carton's probably right in saying that Defarge will one day end up going to the guillotine himself. After all, this is what happened to many loyal revolutionaries after the Reign of Terror took hold, and the radical Jacobins set out to destroy their enemies.