What if...

[Dave]

...You spare and befriend everyone, EXCEPT Papyrus?


Or only keep him alive in a Genocide run? What happenes?

[Lunarluck]

As for the 1st part, you can't befriend everyone if you kill Papyrus, period.
Now the 2nd part, it ends your genocide run

[Dave]

Originally posted by Okami Chibiterasu:

As for the 1st part, you can't befriend everyone if you kill Papyrus, period.
Now the 2nd part, it ends your genocide run

Oh.
Well.




Okay.

[He Who Is Vile]

Ultimately, killing Papyrus will force you to a Neutral route if you were trying to go Pacifist. Sparing Papyrus on a Genocide route will force you back to Neutral, except Sans is removed from everywhere else in the game until the end where he comments on your actions you dirty brother killer but won't actually fight you unless you met every other condition for the Genocide route.

[brickey.8]

If you kill Papyrus, Sans vanishes from everywhere until the judgement, and has something extra to say at that point. (He's not just avoiding you either, Undyne says he's nowhere to be found). Also, if you kill ANYONE Undyne will refuse to become your friend, and if you aren't friends with her you can't do the Alphys date.

Sparing anyone on a geno route shoves you back into neutral, though you'll still get called out from Undyne for all your murder and will get a bad ending.