Well, it's more complicated than that, but Papyrus can think about it however he likes. Realistically, Sans just feels like Papyrus is the Harbinger he is most personally responsible for dealing with. It's a family thing.

But, Sans doesn't get much chance to elaborate on anything, because Papyrus is immediately reacting with utter outrage, and finally going for the attack. Sans pulls what is probably the expected response, and teleports out of the way, placing himself behind his brother instead.

Then, he summons up his own bones, infused with angelic energy and glowing a bright gold. He brings them down in a cage formation, trying to trap Papyrus rather than hurt him. Meanwhile, he puts some distance between the two of them on wing, though he knows the physical limits of this dimension will come into play if he pushes it too far.