Fortunately, although Frisk isn't aware of this quite yet, Papyrus got a significant boost in strength when he arrived on Vaikuntha. The poor doofus probably would've bit the dust ages ago if he was as weak to killing intent as he was in the Underground.

"Yeah, I think so. Just follow the sound of rattling bones, shouting, and very loud science and you should be able to find him pretty easily."