Ten employees are locked in their studio with a killer among them, slowly dying one by one. The mere concept is terrifying.
The deaths so far have all been this in some way:
Michael is poisoned and dies slowly.
Various mousetraps completely chew up the Gavins.
Chris takes several arrows to protect Burnie and dies the slowest thus far.
Burnie gets stung by a scorpion and dies immediately.
Gus opens a door for the killer and is skewered with a pitchfork.
Adam is ambushed from behind and strangled slowly by computer mouse wire.
Lindsay is mauled by a wild puma.
Miles is glued to a treadmill and forced to run at increasingly faster speeds until they die of exhaustion.
Barbara dies from explosives planted in their suit.
The killer: a nondescript person in a skull mask and Ryan's outfit from Let's Play Grand Theft Auto V for anonymity. They are quiet, ruthless, have a sick sense of irony in how they kill their victims, and have been at it for a long while.
The incriminating photos at the start show that everyone at the party has been a killer before (except Ryan, who just has Edgar in the hole; Michael, who set the blackmail up anyway and might actually be a killer after all; and potentially Miles, who doesn't get a photo). Everyone has killed another employee in the office; Lindsay killed Kdin, Barbara killed Blaine, Gavin killed Meg (his girlfriend)... the list goes on.
Michael's horrified look at his poisoned glass is rather chilling.
In the fifth episode, Ryan wakes up in a hole. A bit of Fridge Horror kicks in, when you put yourself in his situation; First, you've just had a vivid hallucination of murdering yourself, and promptly passed out (what the hell was in that Diet Coke, by the way?). Second, you wake up, groggy, disoriented, and trying to figure out your hallucination. Third, you find out you're trapped in a small place, covered with glass, with no idea how you got there. And then, you accidentally piss off one of your only hopes of getting out, and now, you're trapped, even morose than before.
Ryan's little series of breakdowns while his oxygen ebbs away in Episode Six. Because it's meant to be a Fake-Out Fade-Out you could really believe you're watching him try to distract himself from his slowly-approaching demise.
Despite looking ridiculous and being mostly Played for Laughs the thought of being chased down and cornered by a puma is actually terrifying.
It may have been listed above, but Lindsay's death deserves special mention here. In the shortest episode of all, a puma gets out and, one by one, tracks down the employees: Miles hides behind a locked gate, Barbara seems to scare it away with her cat piano, Ryan is backed up against a wall, and then Lindsay comes in and distracts it... and it promptly pounces her, and offscreen, devours her. Except we see her blood, severed limbs, organs, and decapitated head fly onscreen during this. Easily the bloodiest and most unsettling death of the series as of that episode.
What makes it even more terrifying, is the fact that aside from Michael's death, every death was Played for Laughs. This one isn't.
The In Memoriam part of Episode 7 is just a little bit unnerving. Especially once you learn who the murderer is...
We learn in the final episode that the "100% accurate Creeper suit" seen in the list of weapons is actually a real thing, and was made by Ryan for Gavin. Whether he made it for him as a funny gift or a murder attempt is left up to the imagination...
And the way it's used to kill Barbara.
The killer's monologue while Ryan was tied up. Barbara planned everything, in order to incriminate Ryan. From killing off the other two Achievement Hunter members first, to planting clues, to using Ryan's GTA costume so the police would think Ryan was the killer. Really, how much thought went into it?