Betrayer + Killer my take becuz of the reference of Violet from Blood World.

Phase 1:

Sans goes killing and torturing other monsters and humans to due to his kind, and now they deserve to die every single one in the forest to fight you.

Phase 2:

He had schizophrenic and after he's going to the school to kills some students and starting hated himself to a sadistic scumbag and burn the school after the rain starts after you slashed him.

Phase 3:

Sans encounters you in the train station after he vomits some black substance, bringing out the gasoline and the flamethrower to put you down and sacrifice himself together with you due to his suicide.