Just went back, I agree it is quite ambiguous might be a translation error or something since she definitely had no intention of harming cassis and his knights but take my description with a grain of salt lol
I'll just assume that the butterflies are there to eat away at the monsters' carcasses after the Pedellian knights slaughter them so that people won't be suspicious to find dead monsters at the northern border since Dion wasn't ordered to go there (￣ヘ￣;)
Ah that makes sense I'll believe that too then (￣∇￣")
And honestly can someone explain the ending of chapter 26 for me? Roxanna said she can't have any evidence of her assisting in the escape but the sentence was quite ambiguous not stating whether the poisonous butterflies she ordered to go to the black forest were supposed to keep Cassis safe or kill him and the Pedellian knights hmmm