Evil's Wave

In 2014, a serial murderer preys only on women in Kawasaki. The victims' are hanged to death by their necks. The criminal investigation is difficult due to the lack of clues, but Detective Iguchi finds a man named Naoya Nogi.It turns out Naoya Nogi's true identity is that of Masato Yaginuma. He was the victim in a case where a mother and son were kidnapped. What is Naoya Nogi's motive for the murders?