While it's unclear how much autonomy Phone Guy has over his phone calls in UCN, the Adaptational Villainy of the once-peaceful animatronics (i.e. from Freddy Fazbear's Pizzeria Simulator) is justified with implication that the player character is William Afton, the main source of conflict within the series. Even if they were willing to leave other protagonists alone, they will not be as nice to a child serial killer and tormentor of spirits.
Likewise, the inclusion of his Pizzeria Simulator incarnation could be a mockery of his inherent cowardice, killing people children, even to further his research, or just because he felt like it. Scraptrap in this game, billed simply as Afton, strikes exactly once and then leaves whether he was successful or not, as if he just gave up at the slightest hint of resistance while picking on someone his own size.
Scrap Baby also makes no more than one attempt to attack per night. Like father, like daughter.
Then there's the fact that Springtrap is an another animatronic used to kill and hide someone, even if the victim was Afton himself. It's unlikely the kids are in hell with him, so the suits are likely possessed by demons or somesuch.
Most of the Marionette's lines support the "William Afton in Hell" theory, especially how she enjoys William Afton's helplessness and how she used to be afraid of him, but isn't anymore. The Marionette never had any reason to fear Michael (if she even knew who he was to begin with), but she very likely feared William Afton at one point since he was the one who killed her. It's also clear from the rest of her dialogue that she's much more intelligent and aware of what's going on with the UCN than the rest of the animatronics, and it's very unlikely that she would make a mistake as basic as confusing William and Michael Afton for each other, especially since she's likely seen them both as Lefty during FFPS. But one line "I don't hate you, but you need to stay out of my way." seems to contradict this. After all, she has VERY good reason to hate William Afton after everything he's done. However, the line makes a lot more sense when you think about it in one of two contexts either she has finally let go of her hatred but enjoys seeing William Afton pay for what he's done despite that, or she was being sarcastic about not having any more malice towards Afton.
This also explains why Rockstar Foxy is willing to help the player out, even though they're playing as William Afton. It's not because Rockstar Foxy has any good will towards Afton; in fact, he's just as willing to kill Afton when he's in a bad mood. Rather, Rockstar Foxy understands that Failure Is the Only Option for Afton: he will be doomed to die at the hands of his animatronics time and time again, for all of eternity, regardless of whether or not he survives each night. Rockstar Foxy isn't sparing Afton: he's prolonging his next death.
Combined with Fridge Horror, but the Toy Chica cutscenes show seven victims of Toy Chica, who is clearly a stand-in for William Afton. But we only know of SIX, Charlie and the 5 MCI victims. This could very well mean that there were SEVEN victims of William Afton, and that this unknown 7th victim is "The One You Should Not Have Killed."
Given some of the lines the animatronics say when they catch you, a common belief that one of the child spirits is trapping William within UNC. Already terrifying on its own...however, it's not just one person who says these lines. It's several. Best case scenario, some of the other children decided to help this one. Worst case scenario? The children were all dragged down to hell with William.
Or it's not actually the children speaking, but lines manufactured as if it really were them as an additional punch to William—making it seem like he's being tormented by those he tormented before. Alternatively, if it really is the children, getting revenge on the one that murdered them would be heaven for them so that's what they got as a win-win.
There is the fact that while William did murder plenty of people, so did the kids. They went after the night guards as part of their retribution, even though pretty much all of them were innocent. It's likely that, if the animatronics in game are truly the Kids, they're also in their own Ironic Hell. They can never reunite with their families, they can never go to Heaven, they're stuck in the same bodies they possessed when murdering the night guards and forced to torment the man who truly killed them for eternity. Though it might not be as bad as it seems: they get to be the gatekeepers of William's Hell and ensure that he never escapes no matter how hard he tries.
Orville's secret quote directly implies that the vengeful spirit is personally interfering with the ending of Pizzeria Simulator by trapping William in the Ultimate Custom Night rather than letting his soul get released. However, Cassette Guy was deliberately trying to cut William loose to send his soul to Hell, so if William got trapped instead, then he's not in Hell. And it might very well be that the only way to escape the Ultimate Custom Night is to beat it, so if William beats the Ultimate Custom Night... well, Purple Guy's spirit is now free to wander.