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Nightmare Fuel: DEVILMAN
This series is one of the earliest horror manga out there. As such, it manages to have quite a bit of horrifying imagery throughout its run. But for some specific examples...
Jinmen. Check out Tear Jerker for what he's done, but the sheer glee in which he takes from it, the taunting, the way he defends himself from Devilman...it's simply horrifying.
"Compared to you, I'm like a cherub! ... Humans are my nourishment. For a demon, it is not a crime to eat living beings. After all, humans eat cows and pigs, right? However...killing other living beings is a sin! Isn't it? That's why I ate them without killing them!Without Killing! Without Killing!I Ate Them! Kah Kah Kah Kah! Don't you think they're happy to have been eaten by me?!!!"
the faces of those he's eaten appear on his shell, still alive, and still conscious until he's done digesting them.
When an Angry Mob descends on the Misimaru's home...and tear both Miki and Tare limb from limb (Tare first, in front of Miki) cements the idea that as evil as the demons are, Humans are Worse.
A spider-demon burrowing out a girl's face. It had controller her mind and over time, ate it away completely, burrowing into her head until the body began to slowly rot.
Between the amount of Made of Plasticine and the extreme downer ending, it's already pretty dark. It enters new levels when Plucky Girl Miki is torn apart by an angry mob and in some adaptations, her head put on a pike. Her little brother too. The hero arrives far too late, Akira's ensuing Roaring Rampage of Revenge is off panel, but the ground is covered in blood. The last panels Miki appears in has Akira weeping, the last bit of his humanity dead, while clutching her severed head. Then it gets worse.