To begin with, this is a disbelief take. I did see that Spamdoor had a similar concept (called papyrus retaliation, which is a great animation, check it out), but it isn't where I'm going with this AU (I guess it's more of an Alternate Timeline than an AU, but I'll stick with AU anyways). I had a whole different idea and ending. My idea is to explore a scenario where papyrus gets into the royal guard due to your rampage and killing sans earlier than usual. Here is some dialogue changes I came up with.

This is the first encounter with Undyne.

U-UNDYNE! I'M HERE FOR THE DAILY REPORT.

O-OH... THE HUMAN? WELL, REGARDING THEM...

...HUH? DID I FIGHT THEM? WELL...

NO... I DIDN'T.

S-SANS FOUGHT THEM... H-HE...

...

W-WAIT, REALLY?! WOWIE!

ALRIGHT THEN... I'LL BE GOING NOW!

...WHAT? YOUR HOUSE? OKAY! SEE YOU THERE.

(papyrus leaves and the scene is the same as in the game.)

Well, that's that then. I'll be working on this concept more, so stay tuned if you're interested.

Peace out.