Itomo Tayasuku Okonawareru Juusansai ga Ikiru Tame no Oshigoto

The work is a crime story that portrays a 13-year-old serial killer, Shoutarou, whose job is to kill. A young man, Akito, whose sister was killed by a small group, one day met with Shoutarou and finally helped with his work. After learning about Shoutarou's work, Akito asks him to find the people who killed his sister and get rid of them.

(Source: Natalie, translated)