What he left unsaid was his main reason for not banishing or outright killing the practitioner of the dark arts. He was technically Shireen Baratheon's closest and oldest living male relative. He felt obligated to take care of her, now that her parents were gone, and she would at best get used as a figurehead to further someone else's ambition if she lived. Her parents were dead, her father's bannermen were all dead or had deserted Stannis, and she was lost somewhere in the northern wilderness.