My own little spin on the Disbelief AU to go with the image. You spare Paps, and date him, but kill everyone else, even sans. Flowey does his little thing, but the souls all gravitate elsewhere, back into the castle, where Paps stands and watches you with his own two sockets.
"Eye Believed in you!" He cries and laughs. He drops on his knees and grips Sans' coat tight in his hands. You fight him again, he starts quoting his friends throughout the battle. If you spare him, he will stop for a moment before hardening himself, telling himself he should have attacked you from the moment he saw you. If you try and kill him, he will stop at one health and remain there. The souls enter his body and.. that happens, and Papyrus-ified versions of all of the characters appear on the screen. He uses his blue attack almost all throughout, throwing Sans' gaster blasters at you near the beginning.
The battle is a test of endurance as Papyrus self destructs, loosing himself to madness and eventually killing himself in an attempt to make you reset everything.
FLowey thanks you, as he couldn't have done it better himself.
wow, that was more than I expected to type out.