The first debate over the fate of Louis XVI concerned whether the Convention could try the King at all, and if so, for what crimes. The Constitution of 1791 had promised Louis "inviolability," meaning immunity from prosecution. One of the first speakers was Louis–Antoine Léon de Saint–Just, a brilliant, idealistic, and young Jacobin deputy. He drew on Montesquieu as well as Rousseau to argue that the legislature indeed had both the political authority under the constitution and the moral justification to judge the King.
I shall undertake, citizens, to prove that the King can be judged. . . .
Source: M. J. Mavidal and M. E. Laurent, eds., Archives parlementaires de 1787 à 1860, première série (1787 à 1799), 2d ed., 82 vols. (Paris: Dupont, 1879–1913), 53–56:390–93.