A/N: My thought on why Sakura wasn't able to kill Sasuke. Taking place in canon-verse around episode 214, I think.
So when the avenger and the healer meet again it is not how they expected it to be so long ago, because Sakura is - beautiful and proud and strong - a kunoichi now, not the little scared weeping child he left behind. She had to grow up with him not being there anymore, not protecting her anymore, had to become strong - for Naruto, Kakashi-sensei and Ino-pig and Tsunade-shisou, for herself - and that's exactly what she is when she tells Sasuke Uchiha - because he stopped being her Sasuke-kun a long time ago, hasn't he? - these half-truths and readies herself to eliminate him - kill is such a harsh word - since there is simply no other way to fix him anymore and she needs to be the one to do it.
Naruto has already been through enough and doesn't see that the man standing before her is no longer the Sasuke, his rival and brother, from Team Seven and Kakashi-sensei has already lost and seen enough people he cared about die in front of him to last a lifetime, so it's only logical - and if there's one thing that she has always been, it's that - for her to be the one to do it,right? Right?
So the kunoichi - foolfoolfool -is trying to kill her old teammate - lovelovelove - , while trying to keep herself from crying,and the Uchiha, confused and hurt because dammit, she's supposed to love him, can't believe that that's how it has come.
But they don't understand each other's true feelings, since she is - confused and scared an pleasepleaseplease come back - a fool in love, but is better at hiding it now, and he is - confused and scared and Idon'tknowwhattodoI'mlost - over everything else an Uchiha, so he has always been good at hiding his feelings.
She doesn't know why, at the end, she wasn't strong enough to follow through. Maybe it was because despite how often she tells herself that no, she is no longer in love with that monster, that once was her whole world, she just can't make it stop, or because she ist still not strong enough, but she thinks that it's far more likely that she remembers - a long time ago, waiting for him at that bench, screaming, begging him to stay, confessing - that there once was a boy worth saving and that, even after all the had done to them and her, there still had to be a part of the old Sasuke-kun somewhere inside the man she no longer knows.
Their story was meant to be a happily ever after, but reality interfered.