Talk:Sans/@comment-174.23.14.73-20151011010344/@comment-68.255.3.58-20151013215110

I've seen the Genocide Route when you fight Sans (SPOILERS).

Sans said he tried to go back to the surface, but didn't have enough determination, or he was just lazy. He also seems to suffer from self-guilt, because Papyrus is so innocent and helpless.

If you ask other characters, they say they just appeared by the gate from nowhere. They were accepted in the Monster world. Like Frisk and Chara falling into the world, Sans and Papyrus probably fell. Sans and Papyrus probably still were concious when it happened, but Papyrus died and soon Sans followed because he gave up. But because they might've been HUMANS, their soul survived and possessed the skeletons again.