The French Revolution is often contrasted to the American Revolution, but the two movements had far more in common than most Americans are willing to admit or are even aware of. Both revolutions were the practical outworking of the Enlightenment rationalism pervading the Western world in the 18th century, both were humanistic in their values, and both were violent; both were also fundamentally godless.
It has been said that the French didn't have any right to rebel, or that they went too far, or that they were all a bunch of atheists. The latter may be the case, but as long as you're going to defend the American Revolution, you must also defend the French, and on largely the same grounds. In fact, the French arguably had more and better reasons to revolt against the establishment than their American colonial counterparts.
French society in the 18th century was still largely feudal, though not in the technical sense. Feudalism rose in the Middle Ages due to the needs of the warrior class for sustainable sustenance and for military protection; a knight swore fealty and commited an act of homage to his lord, who in turn provided the knight with land and serfs, which again produced food both for the lord and the knight.
Such was Medieval feudalism; the French feudalism of the early modern period retained the elements of estate and peasantry, but without a real warrior class left the peasants simply existed to make the nobles' lives easy. Which the nobles in France took as license to make the peasants' lives as miserable as possible. The French royalty and nobility basically had the rights of God over the lower classes, and frequently exercised them.
Constant inustice eventually led the French people to rebel. That, and the rationalist political philosophy of men like Rousseau and Voltaire, whose atheism fueled their ideas concerning man's inherent ability for goodness and his ability to attain perfection through knowledge and education. That the French Revolution was so marked by passion of the bloodiest kind belied the pure reason espoused by its ideological parents.
The guillotine is the single most recognizable symbol of the French Revolution. The cry "Liberté, égalité, fraternité, ou la mort!" ("Liberty, equality, fraternity, or death!") seemed to be the motto used to defend the peoples' rampant lust for execution of the nobility, rather than any commitment or promise on the part of the people themselves. In other words, let any heads roll but our own.
None of this is at all pleasant. It's all quite bad, in fact, but it's not a whole lot different than what happened in the United States a few short years previous to the outbreak in France. The main difference was that there were no royalty or nobility directly at hand on which to perpetrate atrocities; they were all far away in England, and that was part of the problem in the first place.
No guillotine, however, doesn't mean the American Revolution was justified. Its two most stalwart proponents, Thomas Jefferson and Benjamin Franklin, were Francophiles (lovers of all things French), and Franklin actually went to France post-Revolution to see how they did things. Both revolutions, after all, had the same root: the Enlightenment rationalism that put human authority over divine authority, and thus rendered the divine right of kings a moot point to be abolished and destroyed.
In the end, the French ended up saddled with Napoleon Bonaparte, a brilliant and largely just ruler who was nevertheless an emperor and an autocrat, which just goes to show that anarchy is unsustainable in the long run. Anarchy is itself simply another kind of tyranny, in fact—in the case of the French Revolution, a tyranny of ignorance and bloodshed as untenable as the feudalism against which it was a reaction.