He returned to seek revenge for the rape and kidnapping of his sister. The nobleman met him and tried to whip him, but the young boy forced him to fight. The noble struck the boy through with his sword.
"Oh God What Have I done?"
The boy dies and Dr. Manette goes to see the young women also dying and he realizes that she was pregnant. She dies later that day.
After all the secrets of the Marquis are revealed they realize that they can't trust Dr. Manette.
Dr. Manette also troubled by all this he writes a letter to the Minister Of State. That night he is captured and taken to prison before the letter could be delivered. It is burned in front of him.