Vengeance and Justice were two seperate things.

Sans was a Judge. It was his job, as Determined by the nature of the Underground itself, to protect the innocents of Mt. Ebott's caverns. Every Vulkin, Whimsum, and Moldsmal was his charge, their safety and security his personal business. It was why his power had been stripped away, replaced with the terrible, destructive ability known as Karmic Retribution. The innocent would never face his wrath, for against the innocent, he had no wrath to bear. But the guilty. The evil. The darkness. They would face power the likes of which none would wish to see. He could tie time and space in a knot, blinking in and out of existence at will, seizing hold of an opponents timestream and jumping double-dutch with it should he so choose. Against the righteous, he was nothing. But against the unholy, he was a god.

Was.

Was, was, was, was, was, was.

Sans was a judge. He would ensure justice, the preservation of innocent life in the face of darkness. Strip the innocent lives away, however, and the concept of justice disolves with it. Sans was a judge, but the institution he represented no longer had any charges to protect. The kid had seen to that. Justice was no longer a factor, but as he watched their soul shatter for the hundreth time, he no longer cared.

I will rain hell on you until the end of time, he thought bitterly as the world blinked. The kid was fine again, ready for another hundred deaths, it seemed. Good.

Vengeance and Justice were two different things. Justice served the innocent. It protected them. But vengeance...it just destroyed. At any cost, evil should suffer and scream and burn and die. It was the very embodiment of why he who fights monsters should be cautious. It became the evil it sought to destroy, bringing destruction for destruction's sake

But Papyrus, and nearly every living soul in the Underground was dead.

Revenge was all he had left.