Lore:

During the events of Undertale: Insanity, after he wakes up in a dream, he start experimenting again to make him perfect, mix with DT and Perseverance, he starts injecting himself before he gets energetic and a little bit insane, after he never gets to sleep or lazy, Papyrus was something went wrong with him, but he never happen, after a few months, he finally snapped, his veins were purple-red, before he gets more insanely energetic, and kills his brother, and gain his LV, before he never kill any monster in the underground, before that thing comes back and mimicking and got him prepared for the next experiment, toriel was looking for sans, before he gets out of control, undyne was looking for him too, to back him to normal.