The human child, the one that had killed hundreds and sent the entirety of the monster race into hiding, walked down a long hallway, knowing the end of their journey was near. In front of them, at the end of the hallway, they saw a dark figure. They did not recognize the figure at this distance, but only thought to themself one thing: "Someone else in my way."
As they approached, a voice they recognized spoke to them.
"Heya. you've been busy, huh?"
The child said nothing, but gripped their knife tighter.
"Do you think even the worst person can change? That everyone can be a good person, if they just try?"
"Get out of my way."
"Heh heh heh heh… Alright. Well, here's a better question: Do you wanna have a bad time? Cause if you take another step forward… you are REALLY not going to like what happens next."
The scourge of the Underground laughed for the first time since they arrived. Laughed for a long time. Then took a large, exaggerated step forward.
"Welp. Sorry, old lady. This is why I never make promises."
The child began walking towards Sans.
"It's a beautiful day outside. birds are singing, flowers are blooming... "
The child was almost to him now, their pace quickening.
"On days like these, kids like you…"
As the dirty brother killer prepared to slice, they were suddenly pushed back several feet as Sans' eyes opened to reveal a flaming blue left eye.
"Should be burning in hell."
The monster that looked human felt vibrations below them and instinctively jumped, narrowly avoiding a wall of bones rising from the floor. Before they had landed a barrage of vertical bones was coming from the side, one of them striking the human viciously. As they were reeling from this wound, odd skull shapes appeared all around. Suddenly, their mouths opened and blasted blue lasers at the human. The accused was not prepared for this onslaught and took the brunt of the damage dealt by the attacks. They fell to their knees barely breathing, and looked up at the smiling face of death itself.
"What… are you?"
"I'm just a guy with a bone to pick with you. Here we go."
The once proud and confident child stood there in outright terror. The short, fat, skeleton that used to laugh was no more. In front of him was a facade, a deadly combatant with a bone-chilling smile.
"Well, what are you waiting for? Having regrets already?"
They growled in response and dashed forward with a slash. Sans was out of the blade's path before it could even hope to touch him and he immediately began a counter attack. Large bones carved through the floor and hung down from the ceiling as they approached the combatant. They were able to dodge them, but knew they couldn't keep this up much longer. After another failed attack, more bones came, but the guilty could not move fast enough and died.
After a quick reset, they appeared again, ready for more. Sans was flipping a bone in mid-air above his finger, eyes closed. He opened them as he spoke.
"Oh, back for more I see. His eye glew once more at the utterance of the last word.
The battle went on for what seemed like hours, death after death, reset after reset, never making it. After so many tries, the judged began to be told of a "special attack" awaiting them. After a few more resets, they made it to the "special attack." They were dumbfounded by Sans and his knowledge of the world they lived in, impressed with his clever final attack, the infinity of time itself. However, this worthy opponent only made them more determined to beat him. Soon, Sans slept, and the wolf in sheep's clothing was able to quietly retrieve their knife and approach. As they executed one last slash, Sans quickly dodged with lightning speed.
"Heh, didja really think you would be able-" The knife cut deeply into his skeletal body, mid-taunt.
A smile came to the victor's face. "See Sans, you're just as useless and weak as the rest of them. Especially your brother."
Sans didn't even seem to hear them. He was staring off into a corner and soon spoke
"I'm going to Grillby's. Papyrus, do you want anything?"
The killer looked away with disdain and decided to move on to their ultimate goal. Unbeknownst to them, Sans didn't turn to ash like the rest. He took a shortcut with his last few moments, but it wasn't to Grillby's.
The winner eventually arrived at the King's throne room. King Asgore tried to stall and give small talk, but the nightmare in front of him would have none of it. As they slashed with their favorite weapon, smile as wide as their knife was long, their arm was suddenly jolted and they felt the blade leave their hand. They turned and saw it on the floor, broken with a bone next to it. They turned the other direction to see something truly horrific.
Sans had 6 souls swirling inside of his chest, and a long scar across his chest, still stained with red liquid. He was enormous, the size of a building, with dozens of sharp teeth, each as tall as the human child. Along his arms and legs were spikes similar to the teeth. They all bristled, crackling with what looked to be electricity or possibly lightning blue magic. At seemingly random points on his body, aiming in all directions, were giant blasters, with an exceptionally large one mounted on his shoulder. Asgore quickly ran away, but neither the child nor the skeleton noticed. They stared into each other's eyes, and suddenly, the giant Sans grinned.
"You thought you could do what you wanted, kill who you wanted, destroy everything and no one would stop you. Well, you made one fatal mistake, kiddo. You messed with my family." The right eye flashed rapidly between orange, pink, and purple, while the left eye flashed slowly between yellow, blue, and green. In the corner of their vision, the child noticed that Sans was reaching for a Save point. A message floated in air: "File one saved."
"I'm going to make you pay for every life lost, every drop of blood you spilled. And when I'm done, you will cease to exist. My determination is stronger." As if testing these words, the murderer tried to load, but could not; his ability to reset was gone. All determination they had eroded as they looked into the face of the thing that would undoubtedly destroy them.
Suddenly, spikes shot from Sans' left arm, pinning the child to the floor. Each spike did massive damage, and before they knew it, a laser from Sans' shin ensured they were dead. Sans followed through on his promise, and killed the child a total of 327 times before he could no longer think of other brutal ways. These included lots of bone smashing, impaling, blasting, and occasionally, Sans used their own sharp hands to crush or stab. He kept the save, just in case, and killed the dirty brother killer for the last time. After that, there was only black for the formerly human carcass. Sans absorbed the soul and using all 7, destroyed the barrier. He released the souls back into their containers and relinquished control to the King before returning home. He didn't know if the cut would return without the healing provided by the souls, or if he was going to die, but he didn't care anymore. He had done what he did, and after all, he could still go back. He smiled as he tested his new ability, making a save file and then resetting to it after breaking a glass or something else mundane. Then, he took a nap, the longest nap of his life, as he considered what he would do from there. Probably go to the surface and kill them all.