I haven't read it cause I don't think I could take it, but Killing Stalking is one of the first things that I can think of. One character has been sexually abused by his uncle and one is a serial killer with a mom complex. The serial killer rapes and beats the other character. The author has come out to say that it is not a romance webtoon because so many people ship the two characters. She came out to say that Sangwoo never had romantic feelings for Beom and that he was just a sick twisted serial killer.
There is also a messed up Yaoi manga author, but I can't remember their name. They do one thats called like...The Pigs Bride or something like that.
literally any of harada’s work but here’s one https://www.mangago.zone/read-manga/poison_and_sex/
These are all webtoons, and the fucked up content is mainly manipulation/emotional abuse, but sort of fit the brief -
A Dead Crow's Stare (not as much smut as you'd think, but uke a weakling in zombie apocalypse sexually submitting for protection kind of?)
Down and Dirty (I found this really messed up - kind of like Painter of the Night style soft mind break/abuse situation where the relationship is toxic af)
Legs That Won't Walk (you think he's being saved from an abusive relationship, but boom, he's straight into another)
Banana Scandal season 2 (bottom had a crush on the top in high school and an incident saw their friendship ruined. Now in college, the bottom is still in love with the top, who in turn agrees to enter a dubious bdsm relationship (with unsafe practices!). I like this a lot - the fucked upness comes from the power imbalance - the bottom lets the top do anything he wants because he feels bad about what happened in high school, and he still loves him. Also has an interesting plot!)
Recommend the most fucked up manga you can think of like really fucked up with some confinement, rape , abuse you know all that