Undertale theory - Papyrus was planning to fight you in the genocide run
We all know the story. We approach Papyrus, and he welcomes us with open arms, to return our "hug of acceptance". In battle, he spares us... and we murder him in one hit.
"W-WELL, THAT'S NOT WHAT I EXPECTED..."
Except it is what he expected. In the game's data, a monster's defence gets drastically lower when they spare you, representing their trust and allowing for betrayal kills. But Papyrus is the only monster who's defence goes UP when they spare you, instead of down. He literally has his guard up when he offers you that hug. And if you do spare him, he says
"WOWIE!! YOU DID IT!!!"
"YOU DIDN'T DO A VIOLENCE!!!"
*sweats* "TO BE HONEST, I WAS A LITTLE AFRAID...
BUT YOU'RE ALREADY BECOMING A GREAT PERSON!"
All the evidence seems to suggest that Papyrus was expecting you to attack him. But if that was what he was expecting, why so shocked when you kill him?
Maybe it's because he wasn't expecting you to do it in one hit. I think that Papyrus was planning an Undertale Red, giving you one last chance to turn back and choose a better path, and if you refused he would try to stop you by force... except he never got a chance to. He thought raising his defence would prevent an instant kill, but he was wrong. With his HP gone and his head on the snow, all that could be done was to say "I believe in you!", one last plea to stop this genocide.
OH WAIT I FOUND MORE EVIDENCE FOR THIS.
If you abort the genocide run by sparing papyrus, go to hang out with him, and then inspect his box of bones, he says this. Not in a neutral. Not in a pacifist. Only in an aborted genocide does he tell you about how lucky you are to have not been blasted to pieces by his special attack.
