After a 300 resets of Undertale: Disbelief Papyrus, before he dies again, again and again, sans knows what's going on and he didn't respond, he goes to the lab to find the determination to get more stronger, he was experimenting alone, after he inject something more isolated, he feels happy and and a little bit insane for the stages, he something feels wrong, he snap out of it, hitting his head to make him less fine, but he can't take it, he grew insane and enraged, cracked skull and dust fragments coming out of him to grew more determined, he goes on a killing spree tons of them, including sans and undyne, lost his sanity.