Underworld series - Pardon My Violent Ways

Original title: 暗黒街シリーズ 荒っぽいのは御免だぜ

Toru Sagara, a lone wolf assassin, is imprisoned for betraying his employer, Hanawa. After completing his sentence, he immediately takes revenge on Hanawa, but as he leaves, he is unaware that a boy is watching him. Toru's new employer asks him to kill a competitor in drug sales, but...