On the afternoon of July 26, 2014, 15-year-old Aiwa Matsuo was invited to the apartment of her classmate in Sasebo, Nagasaki Prefecture. Hours later, she was found dead—her head severed, her body mutilated. The killer? A fellow honors student, just 16 years old, who had planned the murder for weeks and showed no remorse. The two girls had been close friends, attending the same prestigious high school. But behind the scenes, the killer—whose name was withheld due to her age—had been exhibiting di