You started a genocide route, in which so many times, Chara got full control over you, so they took over your body during the fight against Sans, speaking of Sans, he was extremely confused and was on high guards. Your, or, Chara’s movements were erratic, making Sans lose track of where you were. That is until you Gojo’ed(sliced in half) Sans.

“gurh!- drops down on the upper half i’m sorry everyone… i couldn’t stop the human… i’m sorry…”

Right as when Sans slowly gave up and dust, ASGORE came in quick and throws the capsule that contain the second SOUL, BRAVERY. Sans quickly catches it and Chara saw this, so they tried rushing to Sans to get the SOUL. Flowey, however, stalled Chara by grabbing their leg and pull them down with his vine.

“Oh no you don’t!”

Chara responded with cutting Flowey in four pieces however, killing him in the process. Then they started running over to Sans’s place again, in which they get stalled AGAIN by ASGORE this time.

“I’m sorry Sans, I should’ve intervened! Absorb the SOUL and release the monster kind from this nightmare, please!”

Chara reached for their knife and reach it, cutting ASGORE into million of pieces, exploding him. And they started going to Sans’s position, YET AGAIN.

“no! with this power i hold… your efforts won’t be in vain, everyone!”

Sans quickly absorbed the SOUL, giving him another chance at defeating the human(yay, he got legs again). Out of the smokes, Sans floated with a gaster blaster behind him, acting as a jet to boost his speed.

“heh… the REAL battle starts now.”

BATTLE, STARTED AGAIN!