So, I imagined HorrorSwap not long ago, and I can’t believe I never thought about HorrorFell.
I headcanon Papyrus as being the Captain of the Royal Guard in Underfell–which usually accompanies a falling out with Undyne. Soooo, in order to avoid this becoming a King Papyrus ending (which I totally want to imagine, too; I’m lowkey intrigued as hell by the King Papyrus endings), let’s go through a neutral run. A few monsters get dusted, likely in self-defense, Papyrus actually spares the human, and then they manage to give Undyne the slip. Alphys dies/disappears, and so does Mettaton. Asgore ends up giving up his SOUL, so he dies, too.
Undyne, mad with grief, refuses to let Toriel take the throne. I’m going to say she goes as far as to dust her. Papyrus and Undyne fight, and it turns out that Undyne’s still as strong and badass as ever–she’s beating the unholy shit out of his face when Sans arrives on the scene. They fight, and one of Undyne’s spears goes through his head. His HoPe may be abysmal, but he manages to survive.
He and Papyrus end up becoming a bit rogue, I’d say. Papyrus isn’t the Captain anymore because he refuses to follow Undyne, and Sans straight-up hates her. Papyrus ends up rallying Snowdin against Undyne, so it gets cut off from the ration supplies. Luckily, humans are still drawn to the mountain, usually by the folklore, and some still fall in.
Papyrus never really considered eating humans, but these are desperate times. Monsters have already run out of food–Grillby jacked up his prices and burned up several monsters that tried to ban together to break into his storage. The Bunny Shop Owner has been dusted, along with her entire family. Monsters can’t eat one another because of the dust situation, so they end up becoming irritable and desperate, and even the smallest provocation can end up in a dusting. Papyrus has actually tried to get them to stop–he’s the strongest monster in Snowdin, as well as the impromptu leader, so his threats carry quite a bit of weight. He needs the monsters alive to help him fight against Undyne’s followers–and there’s way more of them. They keep Snowdin cut off from the rest of the food, and all the skirmishes have left Papyrus with more than a few wounds.
His jaw is partly unhinged, though it doesn’t impair his shouting, and his teeth are now jagged and sharp; most of them never healed from his battle with Undyne. His skull is littered with cracks and impact fractures, and the fingers of one hand are all crooked and gnarled.
And then Sans brought some ingredients out of the shed and told Papyrus that he could make his lasagna again.
Eating humans, along with the head injuries, have left Papyrus a little unwell. He shouts all the time, sure, but now he gets angry at the Snowdin citizens for nothing and is liable to come bursting into their homes in the middle of the night, bones at the ready to punish them for some imagined slight. He often demands that they test out his traps, too, which leads to several monsters getting maimed.
Even so, the situation has improved in Snowdin since Sans started taking it upon himself to hunt. Papyrus is the one that distributes the food, so it keeps everyone in line for the most part; they have to endure his outbursts if they want to eat.
Although, if any of the monsters saw a human, they’d rip them apart while they were still alive.
Sans is even more unhinged than Papyrus, often toying with the humans before killing them, looking for any sort of entertainment he can get. Of course, Papyrus has set up some traps that are pretty interesting to watch; whenever he sees one of them get chopped in half or freeze to death, he can’t help but laugh and enjoy the show.
Although there are still some days when he just gets to the point.
The only thing that really drives Sans is the need to watch over Papyrus and get back at Undyne for what she did to both of them. If not for that, well… he would have fallen down when she cracked open his skull for sure.