The plot goes that Sans is helping Alphys with a device that uses DETERMINATION and the user’s soul to raise their attack and defense sky high, this is to try to find a different way to break the barrier

However it goes wrong and merges with sans’ soul, causing him to become invulnerable. Not only that but it caused him to lose his emotions and become sadistic, like a killer robot. (like the terminator)

He then kills Alphys and leaves the true lab to slay all the monsters to liberate them all

(this is a pacifist run so you and papyrus have to stop sans and save him)