The manga is about a college student and he wants to bring justice when he sees a highschool girl being forced to be a prostitute by other girls her age. He decides to kill the girl in charge so he pretends to be a writer....

It's not sci fi or fantasy and it not a yaoi or yuri manga either... I just can't find it the manga...

Pretty sure you're talking about Tsumi to Batsu - A Falsified Romance.

Thanks! I really couldn't remember the name of the manga so thank you for telling me! 🙂