No. 6/Nightmare Fuel

The birth of the wasp, with the person rapidly aging and ending up as a shrivelled corpse.
- So far it's happened to three people, one who died off screen. Shion's co-worker is bad enough, he collapses back and looks at his rapidly ageing hand in horror. We then get a blurred POV shot from him looking at Shion, and he shakily asks 'what's happening to me?' The second victim is far more horrific. She first notices the black spot on her face in a shop window, meaning that she's able to witness exactly what's happening to her as she rapidly ages and dies.
- Also happens in a freaky scene to a woman at the end of chapter 10.
The pile of corpses in episode 10 and Sion vomiting upon seeing them. Doesn't help that people were actually still alive in that pile, moaning and groaning enough to nearly break Sion.
- Small, horrific detail: the knitting needle Sion absently grabs onto while climbing that pile. Even scarier in that it's never overtly alluded to, as it's overridden by the horror of the climb in general...
The whole concept of No.6 - being watched over night and day by your ID bracelet. Which allows the higher ups even to eavesdrop you. The best part? You are ok with it, since you never knew the other way. Once I wrapped my mind around it I couldn't sleep for several nights.
Chapter 13 features Safu waking up in the correctional facility after being kidnapped and finding herself strapped to an operating table. A man tells her that they are going to operate on her and she cries Tears of Fear in response. *shudder*
Anytime Shion's Yandere side shows up.
Let's not forget the fate of the slumdwellers who are dumped into the room with the dead bodies - if any of them survived the fall like Nezumi and Sion, they are trapped down there with no food or water, surrounded by corpses. Logically, that would lead to rather desperate circumstances.