Wrong in one respect Robespierre was not the first mass murderer in the name of ideology, not by a long shot. However, before him, most of these types graduated towards religious ideology: Oliver Cromwell, Torquemada, and, of course, Mohammed. Robespierre was probably the first to secularize the genre.
semi-secularize. Robespierre seemed to think he himself was a god. And the Cult of the Supreme Being was a sort of deism. The Incorruptible indeed.