Melenchon, for those who do not know him, is an utter moron in French politics. Now we know he is also ignorant about History, because there's ample evidence that the age of Terror was barbaric and without mercy against anyone who may have shown any kind of sympathy for the nobility or the king, no matter their status. And with bloodthirsty guys like St Just, "Sympathy" could be defined as not being a fanatic for the revolutionary cause. And let's not forget the genocide in Vendee against masses of civilians still in favor of the King. The problem is that the Revolution did not stop at killing the King and its supporters, it basically turned into a massive genocide where everyone could become the next target. Hell, even Danton and Desmoulins were convicted to betray the Revolution ideals.
On a side note, Melenchon is known soviet Russia supporter and he probably has the same reading of the Gulags.