Somewhat odd choice of theme here, horror i'd say is best in books or movies. Manga can convey so little for this subject.
Ichi The Killer.
Kara no kyoukai(better anime).
I am a Hero.
Jojo's bizzarre adventure
I guess akumetsu, gantz, berserk and eden count?
Dunno man, horror is a kinda iffy subject. Zombies are hardly scary anymore, vampires are more of supersoldiers now, scary is something i've lost touch with recently. Gore in a general sense is awesome, but hardly scary. Seen pretty much every situation i could think of, the more scary stuff to me per se on gore is the older more realistic and brutal then the newer guts and organs.
But anyway, there yah go, looked thru all the pages and added any i thought weren't right.
Idk what is considered horror atm I love the gory, crazy stories. I'd say shooting with range of my personal fave manga Hellsing. Believe it or not I have read every single manga u have mentioned including the elusive devillady.
Well tell me if you find any. I'm out of manga atm.
just to add this for other people then. Sunkenrock, vinland saga, vagabond, xblade, shigurui, basilisk, dogs bullets and carnage, Biomega, BLAME, The ravages of time, Baki, Lucifer and Biscuit hammer, BAA: Last order, GUNM.
Yeahp thats about all i can reccommend for seinen. Usually i just read new chapters on shonen tho, because seinen takes forever to update. Hell 1 volume of dorohedoro took a year.