when was you in control?

after Chara take over and a genocide happens for the thousand time sans finally snap he killed her so many time that there was only a little bit of determination to rest the time line. but since there was a little bit of determination Chara couldn't take over again. but sans only saw bad in the human that fall if she stay as frisk. and a genocide doesn't happen will sans for give her. but if not how will she show that she worthy for his forgiveness?