The Reign of Terror was started by the Committee of Public Safety, whose members believed that ridding France of anyone who did not support the Revolution was the best course of action. There were an estimated 20,000 to 40,000 people killed during the Terror including Marie Antoinette herself. The killing did not stop until the leader of the Committee of Public Safety, Robespierre was put to death by the very instrument he used to kill those he found guilty of treason, the guillotine.
The Committee ended up becoming the very thing that the French had been trying to get away from. The citizens that it was supposed to protect ended up being its biggest victims. People were killed for any complaint against the way the country was being run, whether it was something as simple as food prices being too high or out right treason remarks. Other members of the committee tried to bring the killing and extremes under control by suggesting it being made more moderate or stopped all together, but only managed to earn themselves a ticket to the guillotine. It took the rest of the committee turning on Robespierre and aligning themselves with his enemies to take him out of power and finally end the Terror long after it was no longer needed.