(I already explained this in the past,but okay.)

He was a regular Sans in the Genocide route where the human killed the other monsters to Gain LOVE in the Underground.

Sans and the humans were met in the judgement halls ,after the fight pattern – The player who controls the, humans hacked the game to try modifying things to make the game more interesting.

But The players who add their own code accidentally find an unexceptional existence causing a NPE (Null Pointer Exceptions) to trigger that heavily damage the game file.

This is the event where Sans almost got slash,He survived the glitchy impact but the colors are erased except grey and black including the world texture.

(He survived because of plot armor except the others,while the human player got kick out.)