More than two hundred years have passed since the beginning of the French Revolution, have we talked too much about it? I think not. As one of the founding events of modern societies, its history is continually disputed. Its complexity goes beyond the simple category of bourgeois revolution and allows to see in this event a real laboratory of experiments, not models, since they do not lend themselves to be imitated, but can inspire the present to imagine - and build - the future. Thus, the present work focuses on a specific period of the French Revolution: the trial of Louis XVI; in this scene, the main character is the young revolutionary Saint-Just. Being the youngest member of the National Convention, he stands in the tribune of the assembly, and he sharply states: For myself, I can see no mean, this man must reign or die. According to Saint-Just, a king is an ennemi that must be fought, not tried. To explore this cut amid the debates of who and how to judge Louis XVI, I present the following route. In the first part, I investigate the biography of this young revolutionary, the paths of his formation and works before the beginning of his parliamentary career. In the second part, I trace a brief history of the events that precipitate the end of the constitutional monarchy in France, and then dive into the main scene, the trial of Louis XVI. I present the legal theses in question, pro and against the accusation and conviction in the former king, the constitutional issues, and I seek to show the radical rupture introduced by the speech of Saint-Just, keeping an eyes in his works De la nature, de l état civil et de la cité ou les règles d indépendance du gouvernement and Institutions républicaines.