What if deltarune and undertale is a battle between the sanses in their respective worlds.
In undertale sans does anything he can to get the player to get the true pacifist ending or reset the game and get a good ending. When you kill papyrus but not the others he doesn't try to kill you because he knows you will reset the game to get a better ending, he only shows up to fight you to get you to quit trying to erase the world.
But in deltarune sans fucks your mom. This enraged me and I went back to undertale to get a genocide run so I could kill sans and then I realized the 2 sanses are sending me back and forth