I thought it was probably a relative. The house of the little girl’s “Aunt Laurie” appears to be a dump, decorated with trash and skeletons, so I think there was definitely a family problem there. I wouldn’t be surprised to see that the cousin had prior contacts with the police or school mental health personnel. Very sad that this sweet little girl had to pay for it.
The boy was born evil, in my opinion. There must be genetic factors not fully understood yet. The serial killer Ted Bundy was disturbed from early childhood. I think there was an incident where he was found standing and grinning in an aunt’s bedroom after he had put kitchen knives in the bed. He was three years old. Can you imagine the woman’s shock when she awakened? Bundy was a supremely evil and sadistic person.
I would look to see if there were previous incidents of assault, or even worse, earlier killings.