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.