[ Papyrus being dressed in casual clothing and wearing his knapsack still probably makes him seem like even more of a corpse that just happens to be walking around.

Oh, this guy sure is making noise and pointing a gun at him. He has enough experience to know what a gun roughly looks like, even if he's certainly not an expert in identifying the exact one. ]

A skeleton monster, of course!! [ Skeleton monster, very important. As far as Papyrus knows, plain old human skeletons don't do anything like him. ]

There is no need to be afraid, human! I, the great Papyrus, will not harm you!

[ Considering that Papyrus is on the business end of a gun, he seems to have a weird idea of who's in more danger here. ]