First of all, when Cassis escapes she's gonna make an illusion with her butterflies where it would seem like he got eaten by them and everyone will think he's dead and that's it.
And about her escaping... Well it's somewhat complicated but she doesn't intend to escape, rather she wants to destroy the Agriche, in the end, she'll succeed only to return later in order to build a new Agriche.
Hope that helped with your curiosity
