Perhaps part of the reason, Sasuke mused in the hospital after the encounter with Itachi, he protected Sakura so much was that she reminded him of a younger version of himself, before the massacre.
She had dreams, hopes. In the time they had been on a team together, he had observed that she rarely had any interaction with anyone, except for her team, and perhaps Ino. She smiled a lot, worried about her precious people, but she was vulnerable. He had seen that in the Forest of Death during the Chuunin Exams.
He didn't want anyone to be the Itachi who destroyed her existence. No one had protected his innocence—at least he could protect hers'.
It wasn't until he was leaning over Naruto in the Valley of the End that he realized that he, himself, had become Itachi to Sakura. He had left her, despite her protestations that if he left, she would be devastated, despite her family and friends. He was the one to ruin her innocence, her perpetual smiles.
But she was better than him.
He knew this because when Itachi ruined his innocence in the worst way possible, he listened to his clearly-insane older brother and vowed to get stronger to get revenge. Sakura had listened and listed the reasons why he shouldn't go, and he didn't listen.
Instead of swearing vengeance, Sakura swore love. Didn't love win over revenge in all the stories?
So that was why Sasuke left Naruto where he lay, determined to not kill him. He was sick of trying to be his brother—this is where he would start. Leaving Sakura with one of the few friends she had.
Because maybe, just maybe, after his revenge, he could heal with love.
I fully dedicate is piece to the SasuSaku community over on LJ. This pairing is fast becoming my OTP (despite I started out with ItaSaku as my OTP).
This entire idea was concieved after reading the ship manifesto. Go. Read it. Ice Empress Zerai fully orders you too. Otherwise she will find you...and, um...
(shuffling of papers)
Make you listen to "I LIKE TRAFFIC LIGHTs. I LIKE TRAFFIC LIGHTS. I LIKE TRAFFIC LIGHTS. EXCEPT WHEN THEY ARE RED."
Hail!