Sans was cold.
He was the Underground's last hope against that murderer-Frisk. He wasn't even so sure that was Frisk. Sure, it looked like her, but he had gotten to know her on different timelines and it definitely didn't act like the Frisk that he knew.
Sans shivered at the thought of "Frisk" killing Toriel, his brother Papyrus, Undyne (somehow, he didn't get how Frisk had beat her), Mettaton, effectively making Alphys commit suicide, and all the other monsters in the underground.
He was hanging out in the Final Corridor, slurping down ketchup and awaiting Frisk. I am ready for you, you dirty brother killer.
Frisk was walking through new home, going to Asgore's place. She was really bored of listening to Flowey's speech about his ability to save. She had to go down the final corridor and wondered how Sans would judge her this time.
As she stepped into the corridor this time, though, she could feel that something was different. It was the atmosphere of Sans's feelings radiating through them. Oh no. I bet he's really mad at me. I wonder what will happen here.
Sans came up with a plan, to give them a really, really bad time. He was just going to kill them ruthlessly and as simply as he could. He gave some short speech, blah blah blah, birds are singing… kids like you… should be burning in hell… that kind of junk, then just flooded the bullet board with bones. Over and over again.
Adam was stumped. He stood up from his PC, went to go get a drink and sighed. Sans wasn't supposed to make the fight impossible…