Finding out you were the one commiting the genocide(probably killing Frisk too, idk, smh, shut up), Sans waited in the Judgement Hall. After you came, he looked at you, like, really really deep in your soul. Then asked.

"why? why did you have to do this? is this out of hatred, or fun of killing my kind? and why, why did you kill frisk? what did they do to you? you're just a sick maniac... i absolutely despise you. i despise your kind, your abnormality... your LIFE. it's nothing, isn't it? you have zero purposes in life beside killing my kind. you should've stabbed yourself, killing yourself. perish."

After roasting the hella out of you, Sans transformed into his more dangerous form: his negative form. The surrounding's colors inverted, the entire corridor turned blue instead of yellow. Sans's colors also inverted. He spawned a yellow bone, and prepared for battle.

BATTLE, START!