@takashichea: She was the one that attacked them after they came out of the fire an explained things. Some said in the manga Sebastian eats her soul to but it did not happen in the anime.
I use this saying alot " No good deed ever goes unpunished." an in a real way the moral of this story is just that. They did dirty work to save the other kids an were lied to an punished for their dirty deeds. Or you can not sale your soul to save others as a broader moral point.