You know, I’ve made posts about this earlier but…
When Papyrus says ‘That’s not what I expected’ what he really means is ‘I didn’t think I’d be killed in one go’
Because throughout all the interactions in No Mercy he sees that you don’t give a FUCK about his puzzles and walk through them, and he does notice that your hands are covered in a dusty powder.
I think he’s got enough knowledge to know that monsters turn into dust
He doesn’t say ‘That’s not what I expected’ because he thinks he won’t be attacked because in someway he does expect it with all the evidence. He’s giving you one last chance to turn your life around…and if you don’t he was prepared to blast you with his special attack as he tells you in the undertale patch update.
This also proves Undyne somewhat wrong because Papyrus will indeed attack one another if things come down to it. He’s not willing to let Monsters die if he can stop it.