I suspect that many of the attempts at social cleansing are natural (though not necessarily just) reactions to other less visible evils carried out over longer periods of time by those in power. Clearly the Bolsheviks in Russia were not happy people... There was a process that had been occurring for centuries prior which made them unhappy enough to carry out a revolution. It is known that Russia's feudal system had been particularly harsh on peasants for centuries.
The French royals were so detached from the misery which they caused that it seems somewhat unsurprising that they couldn't even keep their heads physically attached to their bodies in the end. Their metaphorical heads were already quite detached from reality before the guillotine made the separation literal.