The whole idea of playing a game with your lives at stake. It is said just to pressure people into doing as Decim says (because the guests are Dead All Along) but still.
The whole premise is really unnerving. You wake up in an elevator without any memories of how you ended up there. A mysterious bartender explains that you must compete with another person, who is in similar straits, in a game with your lives on the line. There's no way out, nor any means of communicating with the outside world. Then there are the games that are linked to organs in the body.
In case the guests don't want to cooperate, Decim shows them the incomplete mannequins hanging in the basement and implies that they might end up in the basement if they will still object. It works every time.
The worst part is that Decim's telling the truth. At least indirectly. Humans can't stay too long in the bars, or else their temporary bodies dissolve back into dummy form. We start to see this in Episode 11. It ain't pretty.
The Dart board (and the air hockey station) is linked to the nervous system of the guests - anytime one of them score a point, the second feels a sudden, sharp pain in various body parts. The sound effects and the moving images of the body parts on the dart board do not help.
The ending for episode 2 is entirely made up of shots of Creepy Dolls as they hang from the ceiling and slowly crumble away.
The ending of episode 9, where Decim offers Shimada to break the air hockey pucks that are synced with Tatsumi's organs so he could kill him one more time via torture.
We even see a close up of Tatsumi smiling during the torture!
Hell, the entirety of episode 9. Not only is it revealed that Shimada and Tatsumi had already accomplished their revenge in an extremely bloody manner, Tatsumi is also decidedly not the Reasonable Authority Figure that he seemed. After killing his wife's murderer, Tatsumi becomes a vigilante of sorts, tracking killers and stalkers until they commit a crime, then killing them. Tatsumi was keeping tabs on Sae's stalker and was present when Sae was assaulted. He just stood there and watched, doing nothing to stop her attacker. Shimada is, understandably, pissed when he hears this. Tatsumi continues to goad Shimada, even after Onna tries to calm them down, until Shimada snaps and stabs the hockey pucks. So much blood...
Tatsumi's smile after he kills his wife's murderer and when he's goading Shimada into following him down the same dark path. Shimada smiles like that after stabbing through the pucks, showing that he's well and truly snapped.
Oculus stops being the funny grandpa obsessed with billiards we thought he was. He starts with using his beard to suck from Clavis the memories concerning Nona's plans. Now he knows that Nona is working behind his back and he's not happy about it.
Ginti persuades Mayu into sending a men to the void because it's supposedly necessary to bring back Harada's soul from it. And later, into going to the "hell" elevator to reclaim Harada's soul from the Void. He was lying all the time.
Technically he didn't. Mayu still chose to be with Harada because she felt her life would be meaningless without him otherwise. With scarcely a second thought, she made that decision to be with her idol, and thus paid the price for it. Harada was going to the Void regardless, but she could've been reincarnated had she not mentioned it.
After episode 11, we finally know how the elevators work from the inside. It's not pretty.
