Over 300 genocides passed, Sans would become insane and paranoid due to it, quickly trying to gain LV as he needed a way to beat you. When you fell down, no one was alive beside a few. You managed to kill-steal Undyne when she was low because of Sans anyway. He would be mad, but ultimately continue, however. Sans did the same trick on you when you were fighting Mettaton NEO. Finally, Judgement Hall, where it will end once and for all. (i'm so good at rhyming trust)

"finally. you're here."

BATTLE, START.

"nice to be back, isn't it? you came back all the time just to kill us. over and over. But it's finally time that i bring you to justice. what? didja expect me to be the same exact old useless Sans i used to be? wrong. you're here to be dusted. just like those you done to. it's time for me to end this."

Dust: "it's a horrible day outside. birds are dead, flowers too. on days like these, murders like YOU... will be having one HELL of a MAD TIME!"