Sans trudged through Waterfall, wanting nothing more than to retreat to the door to the Ruins, where he'd tell his worst jokes to a kind, motherly voice. He knew that voice belonged to someone, he knew that voice's owner was kind and pure. It always sounded familiar, even if he couldn't quite put his finger on from where he'd first heard it.
He struggled to ignore the sounds of agony and terror from the dying monsters, the monsters dying by that wretched human's hand. He ignored the horrified looks of hiding monsters as he passed them. Ignored the piles of dust. Ignored the bloodied footprints. Ignored the eerie laughter from the human. Ignored the sharp yelps from other monsters. Ignored the feeling of regret that he didn't do something sooner.
Sans couldn't give in to this feeling of remorse, he knew. If he did, he'd end up nothing more than his own sorry pile of dust. He carefully avoided the flower and the human, and made his way to the overwhelming heat of Hotland. His breath caught in his throat as he passed the sentry station Papyrus had helped him build, and made his way to the Lab.
He pounded on the door to the Laboratory, and to his surprise it glided open. It was dark inside, and he could barely see anything. The only source of light was a dim glow from a screen. Alphys was sitting there in front of it, clutching some sort of cat-human doll to her chest. She looked about ready to burst into tears. Sans realized with horror that she was watching the fight between Undyne and the human.
"Alphys," Sans grunted. The frail lizard looked up in fright, catching the eye of Sans.
"H... hello, Sans," her voice wavered, but she stood up anyway.
He wanted to ask if she was okay, if she could handle this, but he choked it back down. He glanced at the screen and saw the human wearing the most ridiculous outfit; some cloudy glasses and ballet slippers. But she was winning, and Undyne looked ready to collapse. He noticed the strong warrior was wearing different armor, and had dozens, if not hundreds of spears floating and aimed at the human, but the small, lithe body dodged them easily. That smile she wore was the same as the flower's. Soulless and evil.
Sans noticed a small, yellow creature behind Undyne, watching the fight with terrified and furtive eyes. The child that would go with Papyrus to set up puzzles and traps for humans. He swallowed back anguish as he placed his gaze back on Alphys. Her eyes had retreated back to the screen, and she was squeezing the doll so hard now that it looked like the head might pop off.
"Alphys, you've got to call Asgore, you've got to warn people," Sans snapped.
"I... I already called Asgore... but..." tears were running down her face now, and wet blotches scattered across her white labcoat. "I promised... I told Undyne that... that I'd watch her fight... she told me... she told me she would win... win by the second blow..." Sans flinched at Undyne's ragged and hoarse voice, accepting defeat. He almost couldn't bear the sound of her dust hitting the ground. Alphys let out a small sound, almost akin to a dog's whine.
Without another word, she gathered up some supplies; a few plastic rectangles with the words Mew Mew Kissy Cutie printed on them, some Instant Noodles, a picture frame with a blurry photo of she and Undyne in it with over-sized swords, and a phone. The doll was still clutched in her shaking hands as she put her supplies into a backpack and left the Laboratory with the quiet of a mouse.
"Alphys, I-" Sans couldn't finish, the doors slid shut.