The second part of my comment was referring to the historical Saint-Just, rather than Mantel's. In her novel, she has him as Robespierre's bad fairy (I picture it like those classic Warner Bros. cartoons, where a tiny devil and a tiny angel pop up on Daffy Duck's shoulders! Desmoulins being the good angel...). In the TV programme, she had a slightly different take - now, he's an over-indulged child.