Manga does disturbing things a bit better then anime,the gantz manga is pretty long and awesome.
You can also try the Bezerk anime,the manga is better though.
My favorite horror type manga so far are:
Gantz
Claymore
Berserk
Dance in the Vampire Bund
Princess Resurrection
Neighbor No 13
Fourteen
Elfen Lied
Uzumaki (this one is extremely disturbing)
Parasyte
Bio Meat (this one is pretty good,there is a number 2 as well)
Shingeki no Kyojin
Gyo (also weird)
Terra Formars (wish they would release new chapters,it's starting again this year)

