Mommy Bellatrix
Helena Bonham Carter most likely had no idea how much pleasure it would be to play this nasty character when she was cast as Bellatrix. It’s also likely that she was unaware of how helpful playing this role would be for managing her home.
It appears that there is more to the terror of Bellatrix than just the movies. Billy’s pals probably didn’t know what was going to happen, but at least they now know to act appropriately when they visit his house.