Jean-Francois de Morangias: So tell me sir, do they speak of the beast in Paris?
Gregoire De Fronsac: Speak of it? They're already singing songs about it.
Geneviève de Morangias: Instead of singing songs, they should be saying prayers.
Sylvia: Do you know how Florentine women ensure their husbands come home? Every morning they slip him a slow poison, and every evening the antidote. That way, when the husband spends the night away, he has a very bad night.
Gregoire De Fronsac: You needn't resort to that.
Thomas d'Apcher: And you, Mani? Which one will you choose?
Gregoire De Fronsac: Mani doesn't use firearms.
Mani: Too much noise, too much smoke, bad smell.
Mani: All women have the same color when the candle is out.
Answer: She wasn't really evil. The whole movie is a metaphor for the shockingly bloody French revolution. The fact that (at least towards the beginning) the beast usually only kills women and children is a nod in the direction of the countless innocent (women and children) who were put to death in its course. The healer's daughter is sort of an omnibus of the French peasants' lust for blood. She's not evil, she's just crazy - the mad child of revolution. Sorry for the lengthy explanation, but if one goes into it without knowing, it can be a bit confusing.