Vexen and his mother getting into an altercation. Vexen losing control of his emotions and committing matricide.
Mama Vexen would be placated by the sight of the little girl, then say how sweet it is that Vexen's found some responsible friends. She'd invite the twins and Mia for dinner, and over dinner, she and Vex would get into a fight about how she keeps trying to suppress Vexen but was really just trying to protect him, but he doesn't need to be protected, then she'd say something along the lines of if he's boffing either of the twins, he'd better be using protection, and then Vexen would snap and kill her.
But not with ice. Steak knife, or possibly a potato peeler.
Alrumia would piss himself if Vex did that
Instead of throwing the body into the lake, he stuffs it in a bag and insists on bringing it back to his and Zex's place. Then he skins it, tosses it in a frying pan and feeds it to Xigbar. Letting Xigbar out and putting up a wall or something so he can use the bathroom and whatnot but doesn't get close to the experiments. So he gets to use the closet, the bathroom, and maybe half the bedroom. Vexen could rip out her larynx and feed IT to the garbage disposal. Then deep-fry the rest of her body and feed it to Xigbar.
Vexen's mother belongs to a book club. The book club was meant to meet tomorrow at her house. The other people in the club notice that she's not there, and they see signs of a struggle, which Vexen in his hurry didn't clean up. The members of the book club start speculating as to what happened. Someone jokingly suggests that her "daughter" killed her.
(They knew Vexen as a child and knew that his mother used to dress him up as a girl. Some of them, anyway.)
One of them freaks the fuck out and calls the cops.
Lexaeus would also find the leftovers tasty but Zexion would pass. Marluxia wouldn't let Mia have any.
Xaldin would drop by unannounced looking for his roommate. He followed Marluxia, Alrumia, and Vexen's car; he recognises two of them from the club. He made a list of people Xigbar slept with before he went missing, and this happens to just be a stroke of luck for him.
Xaldin hired various hitmen to take out the people on the list. One of them also happens to be Vexen's cousin, Axel, who also has a genetic mutation. Axel decided to visit his cousin in between hits… he notices the very disturbed book club… the fact that his aunty isn't there… the fact that the dinner table is a mess. He asks the ladies if any of them know where Vexen lives.
Will he side with his boss, or side with his cousin?