Guilt and regret that someone's dead, regardless of how they'd hurt him... Papyrus just sorta nods to himself. It makes sense to him, to feel like that... even if Sans is beating himself up about it more than he needs to. But that's Sans for you; in public he laughs off everything, and in private he's always hurting himself in ways that make it harder to do anything.

"It's too bad, we couldn't convince her to be friends," he says, as if that was why Sans is sad. "But... she just wanted to hurt people. It was hard enough, keeping those guys from killing everybody."

He'd probably feel friendship for the the human even if he could remember them killing him.