I like that it was the mother that broke down - a woman who showed no lover for her daughter at any time, no empathy, no concern. The woman that brought Melissandre to Stannis in the first place, and believes in her religion fanatically. And yet, what they were doing was such a horrendous act, so inhuman, that she was the one to try to stop it. Having that happen made Stannis seem all the more cruel and evil for his actions.
Yeah, for reals, I didn't think she would have. Very unexpected.