Lore:

Sans was mentally odd, and goes to the doctor to need help, before something went wrong that gaster possessed the doctor to inject dt, but he just escaped and ran away, and HE smiled, sans went goes back to sleep, and just having a nightmare to hated him and tortured him to no longer be with his side, and now this led him delusional to his perverted desires, he is now 25-50% insanity, and he's isolated, then he grew determined and purify to find out why, this spiralled into led his destruction to a mass violent killing spree between humans and monsters, lost his balance.