Sans has to do the unthinkable. The human is dead, and so is every other monster in the underground, all that are left, are Papyrus, Sans, Asgore, and Flowey. Papyrus started to remember the time lines, and he began to think "What matters anymore" He gave up. With the words of Flowey in his head, and the anger in his heart that he can not fix anything, he goes with the solution that Flowey gave him. Gain Love, gain EXP, gain DETERMINATION, by termination. To finally set everything back to normal.
One month before Frisk falls into the Underground a different human falls, someone that is not from this world, but from ours, and now they have the power to change the world as they please. What will they do, will they do the genocide route and become a megalomaniac? Nope, they want to be everyone's friend and get their version of a perfect ending.