http://www.mangago.me/read-manga/shiiku_gakari_rika/
I'm dragging you to hell with me
Definitely "The Orc Bride" :'D Though I only read it because people on mangago kept talking about how sick it was, so I got kinda curious... yep, sick isn't even enough to describe this shit (⊙…⊙ ) I don't think that's what you are looking for with this post, it just came to my mind when a read "the most messed up" ^^"
http://www.mangago.me/read-manga/shinkai_no_venus/ that's the most messed up for me
Ooooph~ Everyone recommended good ones but this one is also pretty good, except that it's still ongoing and updates like once every year or so.
http://www.mangago.me/read-manga/acid_town/
http://pururin.io/gallery/5001/cannibal-teacher
I've read far worse, but it's still pretty good.
Junjou romantica
http://www.mangago.me/read-manga/hogu_hagyeongsu/
This is a WEBTOON though but it's very messed up
What is the most messed up and the least fluffiest yaoi you guys have ever read? (With violence, tragedy and all, you know what I mean?!)