Story is about an agency, there is around 2-3 male adult and one girl who still in middle or high school. They make a secret agency some sort like revenge agency where the client pay them money and ask them to torture or kill the target.

The agency won't target innocent person and always do investigation is the target innocent or scum. Most of the target are the absolute scum , pedophile, rapist, murderer, etc they can't be brought justice towards japan law system because the victim don't have proof to report it to police, even they had ,they usually got light sentence so the victim think they deserve cruel punishment. Some of the target(yakuza, wealthy people) also have connection on the police or such so they can avoid the law system even if the victim have proof.

Is it Dendrobates? I haven't read it yet but the description seems somewhat close to what youre describing

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I love gender bender and delinquent manga

ah no, it was'nt action manga with gun or martial arts and such

the agency also did'nt using gun or such , but the members are strong man, i remember one of them was a deliquent and a boxer , who almost go pro but got into accident. His mother keep supporting him and he love his mom a lot and regret for not being a good son. One day a snatch purser riding motorcyle, try to snatch her purse, she tried to fight back and the guy using a bat , hit her head till she die of bleeding.

The purse snatcher got captured, and into trial, he had commited many horrible crime before killing the mother, and only got lighter sentence of 5 years, the purse snatcher also did'nt show any regret or guilt, after he got released he still do crime.

The man whose mother got killed, then decided to kidnap that guy and torture the guy and after done, he decided to kill him but the other guy from the revenge agency told him to stop, that was the story how he joined the revenge agency.

Found it the title was the scum of good and evil