I think at the very end the original B-ko lets the clone kill her when she realizes that if the clones can live happily together 'like that' then that's better than her living like this any longer. They'll live happily ever after in a way she never could. After all, it's the clone A-ko over there, not the original. It was a clone all along.
Wait, can the clone make more clones? That would, for sure, mean B-ko is the original, not swapped in the very beginning.
If it is clone B-ko at the very end? Running off to live with clone A-ko? At least they won't have to keep murdering themselves any longer.
(I hope we can edit this in case I re-read it and figure it is different)