The sheer cruelty and sadism of the people who are part of the Circle on the dark web. Not only are they participating in murder, rape, and slavery of innocent women and girls but they also get a sick joy out of it and unlike Laura Barns who kept her anger focused on the teens who had a hand in her death these guys not only go after the main characters but also their families and loved ones. Just to drive this point home they kill one of the main character's sick mother while she is in the hospital seemingly just for the hell of it!
Not only that, but the reveal that THE EVENTS OF THE ENTIRE MOVIE was staged by them as one of their twisted "game nights" makes you wonder that there could be hundreds of innocent victims that also gone though similar situations in prior nights, in addition to girls like Erica...
The videos themselves, even if most of it is implied. For instance, one video shows a girl about to get covered in some kind of corrosive acid, but the group avoids watching the whole video.
Even worse, the ending reveals that one of the police officers responding on the scene, and who likely shot AJ to death, is one of the Circle members. Which means he likely is murdering a lot of innocent unarmed people and using his cover as a police officer to get away with it, which like with the stuff on the Dark Web, can be Truth in Television.
Whatever the hell causes the Charons to move like shadows and cause glitches to whatever screen they come across.
The sight of Erica having a welt on her forehead from the trephination, which she peels off to reveal the gaping hole in her skull.
The fate of Matias in one of the endings. He is buried alive in a coffin, and he is left to desperately call out to Amaya for help, but she can't even see what he's writing or saying through the video chat, so he is eventually going to suffocate down there.
