It was exactly the 500th dusttale timeline.
Sans was starting to see what he was doing was wrong.
He felt sorry, but it was too late.
he had killed everyone.
though he did not let his efforts go to waste.
He decided to try his best to kill the human, while feeling sorry at the same time. He decided after this, he will go back to his ways, but papyrus does not agree with this and takes over half the fight.
showing absolutely NO mercy whatsoever
Then papyrus gives up. While sans forces papyrus to stop, then papyrus gives his own soul to the human, hoping to make them happy. NOPE =D