AN: If then that friend ask why Brutus rose against Caesar, this is my answer: not that I loved Caesar less, but that I loved Rome more. -William Shakespeare's Julius Caesar
Sakura tried once to kill him. A poisoned kunai in her hand, and all she had to do was to drive it into the flesh of the boy who had caused her so much pain. She'd failed, almost gotten herself killed for her troubles, for her weakness. Next time, she will be stronger, she tells herself fiercely as she pulls the black gloves onto calloused hands.
If, for some reason, Naruto can't make him face the light... she will have to kill Sasuke. Help to kill him, at least. If Naruto cannot bring him back, that means no one will be able to, and she will be stronger and try to kill him.
Back when she was younger, she would have never even thought it a possibility that he wouldn't return, that he wouldn't come back and make Team 7 whole again. Now she not only saw but also prepared for that possibility.
Maybe that's what growing up is. Because she used to be wide-eyed and innocent, believing that everyone could be saved and nothing was inevitable. And now she is older, jaded, wiser. She knows better; not all can be saved. Just like Rock Lee will never form chakra. Like Asuma's death still leaves Shikamaru disoriented sometimes. Like Hinata will never quite recover from the abuse of her family, even if the stutter fades.
Young Sakura believed one of two things would happen: Sasuke would come back on his own, when his mission was through, or Naruto would force him back, and all would be healed.
Older Sakura knows better.
Young Sakura, had loved Sasuke. Maybe it was a little girl's love–foolish, self-centered, and shallow. It had been love, nonetheless, and love should never be so easily discredited, no matter its purpose or reasons. She had loved him, once.
Older Sakura doesn't know what she feels for him, really. Something moves in her heart. It's like petals flying on the faint spring breeze: illusory, mirage-like. She still cares for him, certainly. Even if it's futile, even if he's changed so much, even if her heart hurts so much when she even thinks of him. She still cares for him, no matter if she loves him or not. (And she doesn't want to think about whether or not she loves him right now. She'll leave the skeletons in the closet for some time, give herself a chance to breathe.) She still cares for him, wants him to live.
But if Naruto fails to keep his promise to bring him back, if it becomes an issue of him against Konoha, she will help to kill Sasuke.
It is not that she cares for Sasuke less, but that she cares for Konoha more.
She sees that now. She doesn't like it, but she understands.