Horror is definitely one of the weaker genre of manga
Here's a few suggestion from what i've read. Some may be just borderline horror
Mahou shoujo of the end (Fantasy + Horror + Splatter, interesting but confusing read about magical girls. Its still ongoing)
Ousama game (More of psychological but it's got lots of splatter if that's what you're looking for)
I am a hero (A real zombie manga unlike HOTD)
Higurashi No Naku Koro Ni (Must try this, i think something like this is what you're looking for)
Umineko No Naku Koro Ni (Similar to Higurashi ^)
Dead tube (Pretty good art, you might like it if you can appreciate depressing stories)
Parasyte (More action oriented)
Saiteihen no Otoko (Lots of splatter)
Trash (Guns, knives, blood and guts)
Monster (A pretty popular series with a good antagonist, you should definitely try this if you haven't)
Ibara no Ou (A short series with a good story, more of suspense rather than horror)
Kamisama no Iutoori (More like a shounen type of horror, if you get what i mean)
Kyuusen no Shima (Horror + Boobs and this manga is what you get)
Deadman Wonderland (This is the farthest from horror from the list i think, but its got tons of splatter)
Berserk (Not really horror, just swordfights with lots of blood and guts)
Claymore (More of a mellow version of berserk imo)
Shuuen no shiori (Lots of text but definitely horror with a mix of psychological, the art might be a turn-off though)