NEVER put Meru in a position where her phone signal goes out of range. She will revert to a ferocious mess of mindless demonic babbling straight from the fiery pits of Hell that would make Satan smile like the Cheshire Cat!!
The true scary part is Kafuka's reaction and how she snaps Meru out of it. Afterwards when Meru looks into Kafuka's eyes she sees KILL KILL KILL KILL KILL KILL KILLY. It is no wonder why Meru thought Kafuka is dangerous.
The end of Episode 5's purifying is a startling vignette that can haunt the dreams of those unprepared for it.
Zoku's first episode is a treasure trove of Nightmare Fuel, in particular Kafuka's creepy "Traumerei" and school lighting fixings turning into the bottom halves of crying babies.
The ending of the hikikomori episode.
Zoku Episode 8 ended with a moment to rival the ending of Season 1, Episode 5. And it's all the more terrifying considering it focused on Kafuka, who has been shown at times to be as homicidal as Chiri, if not even more insane.
Chiri has been letting bits of this slip more and more lately, such as her description of a childhood game called "Meat Doll"
Don't forget the cleaning episode, where she first pins Nozomu to the wall with knives and then tries to get him to "use the dead space" in his brain by trepaining his skull with a drill.
One chapter ends with Chiri apparently killing Abiru with a shovel. The final page has more innocent panels plastered over it of her instead playfully bonking her on the head with a plastic mallet, but the reader can still see parts of the much bloodier panels underneath.
And there was the ending of manga Chapter 84. Both in how Chiri can apparently stretch her neck out to a creepy length, and the "then who was that?" Twist Ending. (It's all been done before, but Kumeta's stark black and white artwork raises the creep factor.)
One episode focuses on using different genre and animation styles. The magical girl pastiche was adorable, but the simplistic claymation was quite frightening, and the "acid" style forced some viewers to stop watching.
The openings to Goku. Creepy, jerkily-animated paper cutouts of characters with paper cutout!Maria crying arms and legs? Sure, why not? Adding weird old-timey effects and DOOR EYES? Sure! Oh, we forgot to include the part where Kafuka is dancing and tossing flower petals on the dead bodies of her classmates. No problem, we'll add that, as well as a heaping of Mind Screw!
Heck, most of the openings and endings qualify quite well.
It certainly helps that several of the OPs and EDs were animated by a graphic studio called Gekudan Inu Curry, which also produced the Witch barriers in Puella Magi Madoka Magica.
The Psycho parody especially the ending where we see Kafuka leaving the bathroom with a knife in hand...
One of Mayo Mitama favorite hobbies is raping dogs; she likes to use small sharp objects like pencils and branches to anally violate them. Actually, just Mayo Mitama in general.
Mayo: When I really like someone, I want to do mean things to them. (she says as she burns down Itoshiki's house with him inside)
The series ending almost takes this series to Puella Magi Madoka Magica levels of Mind Screw, Fridge Horror and general good old-fashioned Nightmare Fuel. Kafuka was Dead All Along and her organs were donated to each class member, allowing her spirit to possess them and implant them with some of her positive genes to keep them (and Nozomu) from committing suicide. Whenever her classmates "saw" her, they actually saw various other classmates. Class 2-He was effectively Kafuka's Hive Mind.
It gets worse. The children are all Kafuka.
The extended ending manages to make it worse to the point of in-universeNightmare Fuel. Imagine witnessing this family that has every single of its members possessed by some kind of demon that just refuse to stay dead, and before you know it, you got possessed by said demon too and married the demon's obsessed husband without your consent, fated to gave birth for more demonic children. Small wonder that the poor Riku Rouko, said witness and readers everywhere are a bit disturbed with the ending.