Well actually, its not an empty threat. Remember that sans was saying if he didnt promise to toriel not to kill you you would be dead where you stand. Now when sans meets you for the first time the max HP you can have is around 30. While its not actually confirmed that KR is Karmatic retribution and that sans only has the effect on bad people, I’ll just go with it being so since it makes sense. Even then people forget the last part of his last real attack. He tosses you around dealing 1 damage each time you hit the wall, this attack DOES NOT have the KR effect and it can still deal up to 40 damage which is more than The humans health when you meet sans. Sure it cant actually kill you, but it would leave the human at 1hp, which is just one hit even from sans to kill you, and it really shouldnt be too hard for sans to hit you once. So no, it isnt an empty threat