What Happens if you kill everyone but Papyrus

If you kill everyone but Papyrus on Undertale, you won’t be fighting Sans because the route you would be doing would be a neutral route, and you only fight Sans in a genocide route.

You approach what used to be Flowey’s patc, and the flower in the middle is not Flowey, but just a normal, regular flower.

After seeing this, you walk into the purple entrance, and the screen becomes black. Sans calls you, and he tells you about everyone disappearing in the middle of the night. Later, Sans puts Papyrus on the phone with you and Papyrus talks about him being the new ruler of the underground and some other stuff. Sans had told Papyrus that everyone was on vacation, because he didn’t want to reveal the heartbreaking truth about what actually happened to Papyrus’s friends.

While Papyrus was on the phone with the human, Papyrus mentioned that he missed Undyne and thought it was hard being the ruler of the underground.

Papyrus was completley oblivious to what actually happened, and after some other talking on the phone, that was the end of that.