if i didnt get the meaning of "messed up" wrong this ig https://www.mangago.zone/read-manga/danshiryou_no_omocha/
This shit is horrifying
https://www.mangago.zone/read-manga/feeding_lamb/
Not good just traumatising asf like why
https://www.mangago.zone/read-manga/classmate_asada_nemui/
There's a manga written by a cannibal/rapist called Issei Sagawa documenting his own vicious crimes, but it's near impossible to find an online version.
Closest thing you can find is this translated website, but be advised that there's *REAL* NSFL images of dismemberment.
https://peggychang.wordpress.com/2010/01/21/issei-sagawa/
Since everyone has already recommended Nishin Masumi's work, I'll recommend something different. It's short, not super gorey, a little bizarre and confusing, but you can't help but keep reading:
https://www.mangago.zone/read-manga/towards_the_darkness/
I'm bored so rec me the most messed up/horrible thing you've ever read