Cass is my second favorite as well, after Queen. However, the choices this update really shined a light on how she really is and what she potentially tried to do to MC himself. For that, I can't reasonably be with her, if the coup is the only way. If we can fix her or cool some of her hatred down, I will. I just won't kill her like she wants or do a coup attempt for something already in my grasp.
I don't blame her for being the way she is as a person though. Her mom was potentially killed by our father. She lost her eye and took most of the blame for it. Her lover that she genuinely liked turned out to be using her. And, she became a complete outcast in the family. She's rightfully bitter and a tad bit broken by it.
I just don't want to be dragged down to her level when we didn't go through the same struggle. She either let us pull her up to be by our side and be happy. Or, she stay in the darkness by herself. I'm not going to risk everyone for her when she's not really willing to do the same and let her brother get on the throne in a partly peaceful manner.