Sans had prepared for the next reincarnation of the kid human in the underworld. He left no monsters nor main characters alive to break the game and hold the vicious murderer in his Snowdin prison forever. Honestly, every bit of the land started to resemble snowy landscapes when covered in ashes of fallen dwellers.
But a different human arrives. They are called Boyfriend, and they love battling, not in the way Sans is used to. If you spare the poor monster during Anthropophobia, both of you will have a chance for a good end.