I'm assuming that since the banner sticky image is I Am A Hero, stuff around that genre is horror. I'll dump my tops. Also not including any Ito, Urobutcher, or Urasawa.
Gakkou Gurashi - Cute girls doing trauma things, zombies
Apocalypse no Toride - Teenagers in a "No chance" scenario. Zombfags are OP.
Mahou Shoujou of the End - Another chanceless scenario. Lulz to be had
Berserk - fag
Terra Formars - Love the JOHJ. Cool future scenario, name is just a butchered "Terraformers."
Zekkyou Gakkyou - Bunch of short stories. If you're scared of these, you're probably in diapers.
Tokyo Ghoul - I shouldn't even have to name some of these
Zetman - Monsters and JUSTICE
Aku No Hana - Revolves around "Fleur du Mal" aka "Flowers of Evil" by a philosopher which states we all have evil within. Follows a boy.
Doubt - Mystery, murder, rabbits.
Franken Fran - It's more SoL of a psychopath who's a master surgeon. Still horror.
Freesia - Man goes cuckoo? Placed in a future where "retaliatory killings" are legal.
Fuan no Tane (+) - Only series that could have topped Ito in pure psychological horror. 2-10 page short stories placed in real life scenarios.
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