IIRC... they kinda already did this story once. Some girl in foster care got murdered at a construction site, and they found a wooden toy sword at the crime scene. Trail led them back to the foster mother's "special" teenage son, who was hooked on a violent D&D -style videogame; kid was obsessed with the game, and the dead girl, and thought he had to protect her like you protect the princess in the videogame. I think it ended up being that the foster mother was an abuser, and killed the little girl. Or something.