Rinjin 13-gou The Neighbor No. Thirteen

Synopsis

In elementary school, Juuzou was tortured by his classmates. Years later, he finds himself working with one of his former bullies, a man named Akai, who has moved into the apartment above his. As time progresses, Juuzou's revengeful alter-ego takes over and he becomes a psychopathic killer. (Source: MU)