No, I don't view it as a mercy put-down. I think she was beside herself with anger. First he leaves her and her daughter with minimal protection, to fulfill this mission of his. Then the daughter gets bit and dies. Then she see's the governor cut off a guy's head and then pronounce "Kill them all" and leads this charge through the fences that is going to do nothing to ensure the security of the group... like he stated previously was his mission, i.e. group security. At that juncture she had quite enough of the governor. I would like to see her and her sister in future episodes.