Lesson in Murder

Masaya is a university student, but he attends a school that's not his ideal school. His days are generally gloomy. One day, he receives a letter from serial killer Haimura. He was convicted of nine murders and received the death penalty. Back when Haimura was committing his murders, he ran a bakery store. At that time, Masaya was a middle-school student and a customer at his bakery store. According to the letter, Haimura confesses to having committed eight murders, but he insists that he did not commit the last murder. Masaya begins to investigate the last murder case involving Haimura.
(Source: AsianWiki)
~~ Adapted from the novel "Shikei ni Itaru Yamai" (死刑にいたる病) by Kushiki Riu (櫛木 理宇).