Insane deltarune conspiracy theory dump. Tldr: I suspect Carol's faction murdered Papyrus immediately prior to the game's starting date. That's why we the game begins then and there.
Undertale shocked everyone when we all realised Frisk was not us, and Undertale contained a thrilling creepypasta-like sub-plot that questioned the boundary between player and game. Well, now the cat's out of the bag. How do you keep exploring those themes when everyone is looking for them?
Starting from the top, ut/dr are postmodern games where the integrity of the medium is questioned openly, and the fourth wall is itself a plot point. It's possibly more constructive to not interpret the ut/dr distinctions between light world / dark world, humans / monsters, and (importantly) reality vs the video game etc as inherent absolute dichotomies. They are treated as such by characters (and us, playing a video game), but it's an art piece critiquing assumed norms (whether they are those of the characters or the players).
Reading it in that sense, aspects of the game are a bit more transparent. Some characters are apparently mysteriously obliterated in the game's code as well as in the in-game storyline. In Undertale this included Gaster and his cohort. This also occurs in Deltarune with Dess, Papyrus, and Asriel. Asriel might seem wrong, as characters apparently believe he is well - but we're not so interested in what people believe. The characters (and us) have pre-conceived notions that are being challenged. The observable condition of Asriel is that he isn't there. If obj_asriel is absent, he is in the same state as Dess, Gaster, and Papyrus. But we know intellectually they all exist, so it's vital to consider that a character not being there is only equally as important as our knowledge that they should be. Even not being there, they are capable of affecting things. Their absence is a valid meta plot point.
The world seems to exist with a dichotomy of light and dark, but this should be questioned. It seems clear enough that entities can travel to and from light and dark worlds aside from dark fountains (bread crumbs, lakes, and ghosts), and that the Light World has the same essential rules of reality: you find save points and things work the same way in terms of control and interaction. The Light World does not have inherently greater integrity, but is easier to control for humans, as Monsters cannot use magic. That's really important.
I'm interpreting the Voice as Gaster, and that Sans, Papyrus, and Gaster are transcendent figures who are uniquely aware of being in a video game. The Deltarune ("MY DELTARUNE") was designed by Gaster. In fact, I think he also had a hand in making the Bunker, and that if/when we enter the Bunker it will be full of Gasterian things. The Dark Worlds are apparently closer to the "supernatural": spirits may inhabit objects, objects gain life, and the dead live again. Garbage noise is also important, and is linked to Gaster. The Bunker produces slow garbage noise. Gaster made the Bunker to experiment with the Dark World.
Gaster's aims are unknown, but he appears to consider the Deltarune "his". To what extent, again, is unknown. Did he create the universe? Is he referring to the specific deltarune on his bunker? For whatever reason, something went horribly wrong. Gaster was obliterated, and nary a Goner to remember him by.
Skip forward a few years. All this time, Asriel and the bone bros are keeping tabs, but now things have changed. If there was ever a time to initiate a Roaring, it's now. Why now? Asriel is away - it doesn't matter how, only that he isn't coming yet - and Kris is on board, only leaving the skeleton brothers in opposition. But Asriel is coming back. So make hay!
The predominance of the Light World is an inherently pro-Human condition. It's no surprise to see lots of general ignorance and mutual isolation between Humans and Monsters in Deltarune.
An incident occurred in the Bunker several years ago. I suspect Gaster fiddled with Dark Worlds there, but some sort of catastrophe occurred. In messing about with reality (or the game's code) Dess was obliterated. What caused this? If my suspicions on Carol's motives are correct, Carol could have subverted Gaster's experiments somehow, but caused a disaster. Could be that Asriel, Dess, and Ralsei teamed up. To fulfil the Deltarune prophecy, they wanted a Vessel: could be the Goner we made, but never use. For the Darkner, I'm looking at Ralsei because he's a liar, cagey, and behaves like a trauma survivor. I wonder if this is when Gaster was also obliterated? It would give Gaster reason to engineer events away from the Roaring if he sees the destruction it can yield.
One way or another, I believe that Dess transformed into the Roaring Knight: her code was removed, except for some lone text strings, and repurposed. She might be dead, as well, but only turns up in Dark Worlds as the Roaring Knight because her spirit is trapped in dark glass shards. Useful, if horrible, and a strategy that could be used on other Monsters.
We have motive, method, and opportunity for Papyrus being murdered.
The dead can be reanimated in a Dark World. Wonderful way to force Sans into accepting one. It's the only way he'd see his brother again. Papyrus is despatched in such a fashion that he is obliterated from the game except a few cryptic remarks as he fades away. One or another beloved object of his could still project him alive again in a Dark World.
Who would Sans ask for help? He can't. Sans is transcendent, and knows about Undertale, and knows that it's the only way he can see his brother. Besides that, his brother was his only ally. Gaster no longer exists. Asgore is a nutcase. Anyone else would call Sans insane. He probably is, to be fair.
Sans is checkmated and the best he can do (or is willing to do) is wink and avert some harm from time to time. He hangs around with Toriel to stop the Knight from trying to abduct her a second time. Instead, Kris and the Knight plump for Asgore.
After Papyrus is knocked off, Gaster is suddenly out of options. His otherwise transcendent kin are snookered. Gaster is forced to get us, the player, to affect things in a more or less desperate bid to impede Carol. Gaster politely proffers our own Vessel but, perhaps, the time for that already passed during the Bunker incident. There already is one!
At this point things develop around Susie, Kris, Noelle, and their friends. Susie feels betrayed by Kris and Noelle, and goes her own way; favouring the Dark World, her new home, she causes the Roaring. The threat or presence of the Roaring causes everything to go sideways. Humans don't accept this situation at all, and they lead a war against Monsters. Monster lightners are thereafter forced to live in the safety of a Dark World (where the distinction between darkner and monster fades away). They wrote the Undertale legend we see at the start of that game.
We don't always get what we want in this world. Kris certainly doesn't. Kris seems to be in the shape of a Human but is a vessel of the sort Gaster would create, but which seems to have developed its own un-Gasterian ideas by being loved. Consequently, Kris loves Monsters, not Humans. Now Kris has to share their body with a soul who always works against Kris' agenda, and (in a pacifist route) is lovely and charming and makes friends everywhere they go (okay but inconvenient - Susie is not on Kris' side!) or is an oppressively manipulative and violent danger to everyone (weird route). No-win. Kris is forced to repeatedly tear out their soul to try and affect events how they wish. Or just drink milk and take phone calls.
Events may be circuitous, not really because of any inherent rule of those games' universes, except that they are games in our universe, and only exist to be played circuitously.
So one way or another Undertale happens. Gaster experiments with the video game and shatters himself across time and space and into Toby Fox's mind. Sans sees his resurrected brother killed repeatedly by a flower. No wonder he's nuts.
Chronology is the unknown bit. Sans, who shares transcendent qualities with Gaster and Papyrus, seems to anticipate events in both Undertale and Deltarune. It's hard to tell which happened first, except that characters in Undertale seem younger by about as many years as Asriel is older than Kris. I have no idea if it's true, but if the ages do match, then the date of Dess' death could equate to the chronological date of Undertale, which would be an interesting coincidence. A violent reality-warping Roaring and Human-Monster War would presumably cause the irreversible deaths of many of the Monsters that we do not see in Undertale.
Gaster is not a self-insert for the author, but does represent a similar self-proclaimed creator figure, whose existence is defined by his creation. We are challenged to interpret this critique of preconceived storytelling in video games as art vs artist as well as art vs viewer. I'm probably totally wrong about everything, but the fact it drives such engagement demonstrates a thrilling elaboration of Undertale's postmodern storytelling.
