Edited, for typos and style. Second chapter, some of the twists that popped into my head begin to appear more... and if you think I'm messing with stuff here, you should see the other stuff I come up with. This is tame and following canon for me. Enjoy.
Chapter 2-Itachi
Sasuke doesn't know. No one knows. And certainly not Sasuke.
Definitely not Sasuke.
It would simply be wrong if Sasuke knew what Itachi did. It would be wrong if he knew that, in the most secret part of his heart, Itachi still loved and cared for his younger brother.
Sasuke would probably have a seizure if he ever heard that.
Itachi wonders sometimes, though. He knows Sasuke could be stronger, could have killed him if he really tried. So he encourages Sasuke, sometimes, when they meet, hurts and tortures and threatens, because even if there's no way Sasuke knows he's still held back by thinking of himself as always less than Itachi, he is, and Itachi wants to find something to make him break that barrier.
But there's no way that Sasuke knows that.
There's no way Sasuke knows why their mother was so careful to teach him just how much chakra to use to pass out from chakra exhaustion, but not die. And there's no way Sasuke knows why he had to keep using that knowledge, training with their father whenever Itachi wasn't home to occupy their father's time with his own training, or argue over Sasuke's. And he couldn't know what their mother had told Itachi, that their father was planning to just kill Sasuke himself if Sasuke didn't die of chakra exhaustion in one of his training sessions soon, because the father that was so proud of Itachi's accomplishments really hated Sasuke, because weaklings—even by comparison—weren't allowed in the Uchiha clan. And Sasuke didn't know that, however much it had hurt to kill everyone in the clan; aunts, uncles, cousins, parents, Itachi had anyway, because it hadn't hurt nearly as much as the thought of losing his brother. And Sasuke didn't know that Itachi would have left their mother alive, except that she'd known why he killed the clan, had known he was considering it, even if she hadn't really believed he would go through with it until she saw him, and Itachi knew that there was no way she would have been able to keep herself from telling Sasuke the truth, and that would have ruined everything.
And that was the real reason that Itachi killed his clan. If Sasuke knew….
But there is no way that Sasuke knows.