@furato It's his job. Cassius is basically the king's personal assassin. It sucks because the king uses Cassius as a scapegoat, so he doesn't have to dirty his hands, but Cassius's family's relationship with the royal family runs deep.
I hope Roselia can find a way to get Cassius, or at least Ein, out of that life and expose what the king is doing.
It sounds like the first arc in the next season is going to messy regardless.