Ciann's motivations don't make a whole lot of sense from the point of view of a ruler, but she's not thinking like a ruler so much as a straight up religious zealot. She's decided that if Arcachon is going to be destroyed she's going to destroy as much of Ohnar West as she possibly can first.
Really, without Dormondt around to tell her right from wrong she's just a rabid dog, but she's got one of the larger networks of spies and traitors around which is really all that keeps her alive. I'm well aware OW probably won't capitulate, but Ciann isn't.
Personally, I don't get the squeemishness about torture, as torture was common troughout the middle and dark ages and everyone should be using it pretty much all the time. Especially theocracies, as conversion and cleansing via torture was understood as perfectly sensible and logical.
Mind you Ciann's goal of killing Tilted Brim has nothing to do with Xarnelf at all and everything to do with an internal religious purge she's got on her mind.
