Sans knew that something wasn't right about the human. He felt it. Their empty gaze, blank expression… DUSTY hands… They killed monsters. How many? He had no clue. On the snow, the dust piles weren't so well visible, but he could tell by the dust on the human's clothes and hands that they killed a lot. Now Papyrus… Papyrus was too innocent to consider this human actually dangerous. Mistaken? Confused? Frightened? Maybe. But he didn't see the danger. And that's what scared Sans the mostly.
As the human was out of sight, Sans grabbed his tall younger brother by both shoulders and turned Papyrus's gaze to his own, his pupils disappearing.
-Pap, I need you to listen carefully now, 'kay?-
He exhaled.
-Uh… What is it, Sans?-
Papyrus mumbled and Sans continued:
-This human… They are dangerous. They are truly dangerous, Pap. Do you see the dust on their hands and clothes? I need you to stay away from them! As far as you can! I beg you!-
Sans clenched his bony fists on Papyrus's "battle body" shirt. Papyrus felt frightened.
-But Sans… They are just scared and confused! They are a human among monsters! They are afraid for their life and probably use violence as self defense! It's completely comprehensible and I will help them get to the right path.-
As Papyrus finished, Sans sighed nervously.
-Pap, I beg you, stay safe! Don't get into their zone of reach, don't fight them and don't touch them! Do you get it? Do you get it, Pap? Nothing bad will happen to you, just listen to me. Please!-
Papyrus was afraid. He was afraid every time Sans's pupils disappear. He didn't understand what it meant.
-Uhm… I… I will be okay, Sans. Stop worrying! The great Papyrus can take care of himself! –
Papyrus followed the human to the next puzzle. Sans's, if he only had nails, would eat them with hands by now. How could Papyrus not see the danger? It was more than obvious! Sans just hoped his brother will be reasonable.