Near the end of the game the player encounters Sans in a long hallway on the way to Asgore's throne room. Sans will judge the player, commenting on how many monsters the player has killed.

If the player has killed no monsters Sans will commend them for getting this far without hurting anyone and the game will be permanently locked into the Pacifist ending.

If the player has killed a small amount of monsters Sans will speculate on if these deaths were intentional, or the player being frightened into self-defense.

If the player has killed a large amount of monsters Sans will condemn them for their cruelty. He is especially harsh if you kill his brother Papyrus.

If the player has killed every monster and boss in the game then Sans will not judge the player at all. Instead he will attack the player in a gruesomely difficult boss battle. If the player dies and returns he will repeatedly taunt and insult the player before attacking again. Sparing him doesn't work until the half-way point of the battle. But if you choose to spare him he will insta-kill you and tell you to get dunked on.