From what I've seen from Robespierre's fangirls, it's not so much "bad boy" perception but rather "goody-two-shoes boy who ends up commiting atrocities partially rooted into this very goody-two-shoessness" which, admittedly, is an intriguing angle to explore. Besides all other things you listed, many also find compelling his relationship with Saint-Just and Desmoulins, which inevitably turns into slash shipping.

I'd say there is a lot of French Revolution fic out there about other historical figures, but Robespierre is indeed one of the most popular ones.