That expression... it's like barely concealed hostility.

That's not a look Papyrus has ever seen directed at himself before, not from Sans. Usually his brother hides everything important, from anger to sorrow, under his distracting jokes whenever possible. Something is not right here, something besides the strange setting and attire.

"What's it to... Those things aren't yours, right?" He doesn't give Sans a chance to answer, continuing to spin an elaborate imaginary scenario. "That's a crime, and somebody's going to be made you broke their stuff! And, if I don't stop you, I'll be your accomplice. How can I bail you out if I'm in the cell next to you?"

As far as Papyrus is concerned, it's obvious he's not going to just walk away. Not when Sans is in such a openly terrible mood, and in need of somebody to keep him in line.