Wow, that's a tough one! Every webcomic I've ever read pales in comparison to all the messed up, weird guro manga out there. The closest I know of are these:
A schizophrenic girl goes on a killing spree.
A young parishioner becomes possessed by a demon, Father Daniel needs to exorcise him with the help of his ex.
This one is discontinued, and not so much disturbing in content as it is in context. A romance story about a girl, who's a cannibal, falling for a guy and trying to figure out if it's love or if she just wants to eat him.
A supernatural horror comic about bodies of missing people turning up out of the blue, with the forensic scientist in charge of examining them not coping well due to the bodies turning up with disturbing supernatural features that only he can see. I believe this one is either no a hiatus or discontinued entirely.
A story about a woman who becomes infatuated with a singer/local celebrity after meeting with her. This comic is completed.
That's all I can think of right now, I second the rec on Nine Stone, it's a pretty uncomfortable comic. I've been told by a few that my comic is disturbing, but it's pretty tame honestly, just upsetting to read if you've ever been in an abusive relationship and have dealt with serious mental issues. (Which even that really depends, even people who've been in those situations can find it an uninteresting read.)
Good luck finding disturbing shit! I'll be looking through this thread myself since I thoroughly enjoy consuming disturbing content.