It looks like a strangely haughty kind of amusement, one that doesn't suit Sans any more than weird decorative swords do.

Papyrus frowns, and the indecision likely shows on his face as he really weighs his options. Obviously he doesn't want to hurt his brother, or watch him get hurt. And who knows what else the Mother can do from inside there, if she decides the agreement she made up is off.

"Well, I'd prefer you stop. Without... physical force. Saying, so long as you're in Sans you won't hurt him..." He shakes his head. "It's hard to want to make a deal with you, when you're, uh, keeping hostages. You're talking through our friends, instead of loudspeakers or strangers, to remind us you can hurt us, right? That's not very friendly. That's actually very scary and unwelcoming! I don't know if I can trust any deal you offer, when you're doing that."