The French Revolution of 1789, while based on the ever-growing popular Enlightenment ideals put forth by philosophers such as Descartes, Voltaire, and Diderot, ultimately failed because the change in regime created a severe power vacuum. This allowed radicals to seize power and sow chaos within France.
This eventually led to extreme violence and paranoia, followed by the re-installation of monarchy, and, several years after that, a dictatorship controlled by Napoleon Bonaparte. Shortly after the fall of the Bastille, the French worked to re-establish documents and governmental structure, including the writing of the Declaration of the Rights of Man and of the Citizen and the Civil Constitution of the Clergy. Both of these documents were specifically aimed at hurting both the clergy and the nobility of France. These documents leveled the playing field for all three groups and removed many of the privileges that the clergy and nobility had previously held.
In September of 1791, the National Assembly passed the Constitution of 1791, which established France as a limited monarchy. Several factions formed in response to this, including the Jacobins, who completely disagreed with the monarchy and thought the Revolution should move forward. After the opposing and more moderate group called the Girondins, who believed that the limited monarchy was necessary for governmental stability, declared war on Austria in 1792, the Jacobins and the sans-culottes, a highly radical and violent group, stormed the Tuileries and promptly arrested Louis XVI for treason.
In 1792, after the National Assembly faced pressure from a lack of an organized army while on the brink of war, and due to the influence of the Jacobins and sans-culottes, the National Convention established the first Republic of France and executed Louis XVI. The Jacobins and sans-culottes then staged a coup against the Girondins, accusing them of being too lenient on the aristocracy, and placed Robespierre in power. Robespierre then used the Committee of Public Safety to focus on perceived political threats within France and began the Reign of Terror by executing between 15,000 to 50,000 French citizens by guillotine.
Therefore, even though the French Revolution started because of optimistic Enlightenment ideals empowering the common man to run his own government, uncertainty and a power vacuum were left when Louis XVI, the nobles and the clergy were suddenly out of power. This allowed for radicals to step in and accelerate the Revolution in the way that they saw fit, which resulted in a lot of chaos and violence following the execution of Louis XVI.