sorry, i might have worded my question wrong. what i meant was, was his full name "louis de saint-just" or "louis saint-just"? what's the correct way to refer to him?
His birth name was Louis-Antoine de Saint-Just. Sometimes he added Léon, which is a diminutive form of Léonard, his grandfather's name. Dommanget said in his article that Saint-Just might have used Léonard-Florelle as a pseudonym, but Quennedey found his source dubious: when she checked Dommanget's citation, she didn't find any mention of Saint-Just.
By 1792, Saint-Just stopped using the "de". This was part of the movement of democratization and de-aristocratization, the same way "citizen" replaced "monsieur":
The omission of the particle had become a common practice of the time. While not widespread, it was not uncommon. It became part of the broader trends of de-aristocratization and de-christianization. It should be noted, however, that Saint-Just stopped at the elimination of the particle. He did not push the change as far as his Picard compatriot Saint-Simon, who, as early as September 20, 1790, before the General Council of Péronne, had completely abolished a name deemed proscribed “by reason” to call himself Bonhomme, in the plebeian manner. However, it is no coincidence that in Year II, Saint-Just changed his name by signing the decrees of the Committee of Public Safety as Saint-Juste (20). This was undoubtedly to better emphasize his commitment to justice through his name. Through the roundabout method of adding a final letter to his name, Saint-Just thus aligned himself with all those who, at the time, changed their names to give them a moral or civic significance.
This is from Dommanget's article, p. 335.
Near the end of the Ancien Régime, the particle was no longer uniquely used by the nobility, hence bourgeois families like Robespierre's and Saint-Just's had it. Some gave themselves this appearance of superiority (which is what it was) like Danton using D'Anton in the 1780s.
Philosopher and novelist Sandrine Berges has a post on her blog translating and commenting with examples on the official rules for the particle.
The reason I drop the particle when talking about Robespierre and Saint-Just is because they made the conscious political choice to drop it.
