Steak said: Papyrus will kill you. He's just incompetent.
Actually, in non-genocide routes he's just trying to capture you. He will stop short of killing you no matter what, while no one else does. Even Toriel can still "accidentally" kill you (if you really go out of your way to get yourself killed). This implies that he has great control over his attacks beneath his seemingly goofy nature (even though he can't really do anything about Toby being himself). Sure, by bringing you to Undyne he'll get you killed, but he's not trying to directly murder you at the very least.
In the genocide route though?
He just straight up spares you from the start, no questions asked. You can even accept his mercy to boot yourself out of the genocide route right then and there.
