Undertale Headcanon
If dust is made of dead skin cells in our world, the human world, then monsters turning into dust when they die would mean that they have no skeletons. (Supposedly, the way their body works is mostly magic-related, so they don’t need muscles or skeletons or all that other useless stuff humans have.) Of course, for Sans and Papyrus, this would mean that they shouldn't turn into dust in death. In the Genocide (No Mercy) Route, Papyrus' death animation is him turning to dust. This would be wrong. This would perhaps imply that it was not Papyrus who died, merely someone working with Sans to create an excuse to punish Frisk (chara?) for their crimes. Sans, in death, is shown bleeding (ketchup, supposedly, as he is seen often drinking it), but never is seen turning to dust. Therefore, you could've killed Sans there, just not the one working with him (Papyrus?).
It’s a little wordy, a little complex, and a bit morbid, but it’s my headcanon.