Well, it depends on what kind of horror you like. For example, "The world is mine" by Arai Hideki is one of the most controversial manga out there. It depicts a lot of violence/destruction and ***/rape scenes that are carried out by the main character, who is pretty much a sociopath. His partner claims to be a normal person, yet he's fascinated by the the destruction and goes around the MC taking pictures of what he does. It's obviously makes you think "WTF!?", and sometimes even scared by the cruelty of the MC. The interesting part here is that this manga is narrated is similar structure Star Wars was, just you're not following a hero.
Another suggestion I have is Dorohedoro (by Q Hayashida). It is labeled as horror for some reason, tho I think seinen is a better label: Has always very gorish action, but yet the characters are quite likeable. You could say it's similar to an adventure manga, with the difference most of the characters have questionable actions, bg stories that make them complex, and at some point you cannot help but to think the plot is pretty dark (you could say that it revolves about the consequences a certain character leaves behind because of his seek of becoming a "God"). There isn't any good or bad side, but there are like three or four antagonistic ones. And it manages to make you laugh. I always enjoy the kind of stories that are more balanced (in terms of elements present).
Himizu is another good option. It is what you would call black humor at the beginning, but it later moves to explore a lot of darker themes. I'm way too lazy as to describe it briefly, so just check it out.
