The French Revolution was undoubtedly a cataclysmic event. In many ways it was the culmination of Enlightenment Rationalism, in others it heralded (to many contemporaries) a journey into a dark unknown. We are still living with its repercussions.
(a) the doctrine of human rights (droits de l'homme) brought to Europe from America, so that for the first time Protestants and Jews were granted civil equality and the slave trade was temporarily abolished.
(b) a self-righteous totalitarianism represented by the Reign of Terror and the attempt to create a pure citizenry. What happened to those who were impure?
'Qu'un sang impur
Abreuve nos sillons'.
[Let the impure blood water our furrows]
The Marseillaise is here referring to the Prussian and Austrian enemy, but the language of purity could easily be applied to those French people who opposed the Revolution. That's why they had to be eliminated.