It's as I suspected then: claiming Robespierre was insane is a 20th century phenomenon. When someone doesn't like him in the 19th century they tend to settle for "evil demagogue." In any case, regarding Fréron's portrait of Robespierre, I don't think he meant to portray him as insane, but I guess I can see how people might have taken it that way if they felt so inclined.

I don't have the Nabonne with me, but I will definitely go back and give it another look when I'm home. It sounds like it has the potential to be unintentionally hilarious (there were, of course, already parts like that in the section I read).