She must have done it

"You were the one who educated your daughter into becoming a beast and now she was eaten by a beast. You might have forgotten the things that she did before Madam Yu became the owner of the village. But I will never forget nor will those mers and families who were ruined by your daughter." 

Sister Ke laughed loudly as she rubbed the back of her brother. She said to him, "Jiu Jiu are you happy? The one who ruined your life is finally dead the justice is served by the heavens." 

"H…happy. Jiu Jiu is happy, no one will bully Jiu Jiu," the little mer hugged his sister with a scared look on his face as glanced at Second sister Rong's corpse.

Even though he did not say anything else, one could see the raw relief on the mer's face.