Hell Dogs: Jigoku no Inu-tachi

Can I kill a person as a police officer!? It's been three and a half years since I became a yakuza, and the infiltration never ends.

Kanetaka of the Higashi-Sheath Association is a gangster with a strong arm who has climbed up to assistant young head. He landed in Okinawa at the command of the organization and killed his target, Kino. That night, Kanetaka vomits violently. He was an undercover investigator at the Metropolitan Police Department! What is the purpose of Kanetaka's actions!?

(Source: Kadokawa, translated)