After so many many genocides, you really took "It's kill or BE killed" seriously. Sans has had enough of you and decided to do his own genocide run. With his 4 LV, he was ready to get even more. After a massacre in the Ruins, and killing Toriel, Sans made his way to Outpost. When Sans was finished, he moved to Outpost. Of course, not without hallucinating and talking to his dead brother's ghost. But after one massacre in Waterfall, he got to Undyne, and unfortunately, she refused to die, causing a delay which was enough for you to kill her and steal the EXP Sans was trying to get. Of course, his brother was angered, and so Sans battled you.

"you. you're here already. you really took that one simple line seriously, didn't you? i'll take this one, boss."

BATTLE START!