This is why you're unredeemable if you kill Papyrus.

To be fair, you can imagine it from the kids perspective. You fall into a hole, whre a flower tells you that everything wants to kill you. then, some big furry creature starts taking you around places, leading you through puzzles and stuff. You try to escape to find a way out, but you start getting attacked by monsters and stuff, and so you feel like you must fight back. You eventually find a way out, but the giant goat creature wont allow you to leave, and so you kill them and run away.

Then this big friendly skeleton shows up and you realize all the horrible things you did.