Zankyou

Zankyou (残響) is a manga series written and illustrated by Takahashi Tsutomu. It was serialized in Big Comic Superior.

Plot
Satoru spends his days listlessly in a factory town. The old man Segawa, a former Yakuza, lives in the room next to his cheap apartment. One day, Segawa tells Satoru, "I'll give you 5 million yen to put me out of my misery." Satoru hesitates, and Segawa sees through to the madness eating away inside him and begins to have a change of heart, and yet...!? A story of the filth of society and the dark path that Satoru takes -- on the edge of sanity -- with a gun and 5 million yen.