Is Matt a sociopathic Smug Snake, or is he just an Obliviously EvilPunch-Clock Villain? Did he really care about helping Harry overcome his social anxiety and meet a girl, or did he (a married family man, if he's even being truthful about that) just enjoy psychologically controlling and living vicariously through the eyes of a single guy? Is he some twisted combination of all of the above? No matter which way you go, he clearly is not a very morally upright person.
Bethany herself; is she completely selfish, ignorant of Joe's pain, or just trying to make the best of a bad situation and doing a terrible job of it?
On the one hand, it was cruel to block Joe and not clarify anything to him. If she had been honest with him about the affair from the start, the whole tragedy would have been avoided — Joe likely would have moved on and been spared years of pain and false hope, her father and daughter would still be alive, Joe wouldn't be in prison for murder and Cookie Joe wouldn't be condemned to hell for over a million years.
On the other hand, she was planning to abort the baby and possibly try to fix her relationship with Joe, until he found the pregnancy test and blew up at her when she said she wasn't keeping it. Whatever your views on abortion and the right to choose, it's obvious that Bethany is angry and upset at Joe telling her what she could and could not do with her body. She never got Joe's apology letters thanks to her father destroying them, and likely didn't know that Joe hadn't moved on and was obsessed over the girl he believed was his daughter — or if she did know, maybe she thought it was better that he never find out about her affair, since by this point there was no way it was going to end well if he discovered her daughter wasn't his.
Moreover Joe's being a belligerent alcoholic could've kept her from telling him the truth about the baby's true father in fear that he would not take it very well.
Bethany's dad. Is he an unlikeable, snobby Knight Templar Parent whose personal petty dislike for Joe led to him torpedoing any hope of Bethany understanding what Joe thought was going on, which ultimately led to the tragedy of his and his granddaughter's deaths when he decided to Kick the Dog by revealing that he kept Joe's letters from Bethany? Or is he a Papa Wolf who feared that Bethany might put herself and May in a violent situation with an unstable alcoholic if he let her read the letters? Or is it all just as case of Unreliable Narrator as Joe is keen to blame Bethany's dad for as much as possible?
Captain Obvious Reveal: The cabin setting is very isolated, neither man has an obvious reason to be there, and Matt is trying too hard to be charming, so it is obvious within the first few minutes that their interaction is a stealth, virtual reality interrogation.
Complete Monster: Matthew "Matt" Trent is a superficially charming sociopath whose actions have caused suffering for many. His day job involves creating digital yet fully sentient copies of people he calls "cookies", traps them in small devices and enslaves them for his clients. When one "cookie" resists, Matt forces her to experience nearly seven months of complete isolation inside the device, driving her mad and begging for work while heavily implying he's done this many times before. Matt also runs an online group where they watch people have sex, but when his client gets murdered, Matt's only concern was to destroy any evidence linking his involvement in said murder. Making a deal with the police to avoid prison time, he creates a "cookie" of Joe Potter and psychologically torments him continuously until he confesses to a murder. When a broken Joe tearfully admits that he murdered his ex-girlfriend's father, leading to the death of an innocent girl, Matt's only response is to callously cheer before leaving him to the police's mercy.
Matt crosses it when he subjects Greta's cookie into 7 months of complete isolation and then crosses it again when he deceives Joe's cookie and turns him into the police.
If they didn't cross it earlier with causally taunting Joe about his crime despite his visible trauma and guilt, the two detectives definitely cross it when they subject Joe's cookie to millions of years of isolation. Irrationally thinking of it as a "fair punishment". Poor, poor Joe.
The idea that the cute guy you meet at a party could be live-streaming his entire encounter with you to a host of sleazy, perverted, Casanova Wannabes. Not only that, but said Casanova Wannabes are coaching said cute guy on how he can get you to have sex with him.
In the same light, what if the pretty, quiet girl you start hitting it off with takes you home with her, only for you to discover in the middle of a passionate embrace that she's homicidally insane, and intends to kill you and herself.
Also, that monotonous life you might find yourself living where every day feels the same and nothing changes for you? Are you just in a rut, or are you really an AI version of yourself stuck in some advanced, solitary matrix, trapped in time, unable to die or do anything?
Rewatch Bonus: All the subtle, interweaving hints at the big reveal towards the end are made all the more creepy upon repeat viewings.
The cops due to their horrible and downright brutal treatment of Joe. While he did murder Beth's father and indirectly killed a child the former happened because he was under great emotional distress and he had no idea about the latter death until he got arrested (and both of these were something the cops probably ignored in order to give him a death sentence). Not helping matters is that on top of presumably providing him with no treatment or help at all, they constantly taunt Joe about it even when he's imprisoned while ignoring the visible trauma and guilt he's feeling. And their treatment of Joe's cookie is even worse since he's a digital being who had no idea he wasn't real and why he was there in the first place not to mention that he was genuinely remorseful of his actions when he confessed. Also the "fair punishment" they gave to Joe and especially to his cookie was too extreme.
Bethany as well. While her family didn't arguably deserve their fate, she starting the whole mess by cheating on Joe in the first place.
The Woobie: Greta's cookie. It's clear from her reaction that the real Greta had no idea what she would actually be doing to herself, with the copy being justifiably frightened beyond belief.
Alternative Title(s):Black Mirror Christmas Special