Before the human arrived in the Underground, Sans was a friendly and lazy skeleton who cared about his brother, Papyrus. After the human reset and performed numerous Genocide Routes, Sans remembered all the routes and began killing monsters for LV, even killing his own brother. This led to a careless, psychotic, and obsessed individual who has shown signs of schizophrenia and depression. Sans is desperate and broken, having gone too far to forgive himself and doesn't see much use in stopping now