He was a regular Sans who, upon learning of what the human has done, aims to stop their genocidal actions by slaughtering all of the Underground to acquire enough LV to defeat the human. Initially plagued by guilt over sacrificing other monsters for power, Murder Sans gradually descends into madness. Especially when he killed his brother, unconsciously creating with magic a ghost of Papyrus that only he can see, this Papyrus will always support him and tell him what he wants to hear.