Sans was has supposed to end the genocide route, he barely even turned around and see it.

While Sans in the lab there's was a dangerous potion inside, its called (blood organs).

Sans tried to drink it, something bad happeneds to see coming his skull, no eyes and no nose above his skull starting ripping his hole in half, melting slippers and eyes, gorey torso.

And he turn to a C O C O O N.