You are so right. It is not the matter of moral judgement or intellectual analysis of ideas. It is one of the basic principles of a sensible approximation to history. She may find Saint-Just childish and ridiculous, and be surprised why his contemporaries did not see him as such (even Desmoulins, who mocked him for his self-important posing, did not see him as childish). However, for someone dealing with history, this surprise, or incomprehension, should be a starting-point for a productive historical analysis. Because what matters is not Mantel's personal sympathies, but how we explain a person in a particular historical context.