He stares at the sky.
Itachi's passing though the area. He'll have to stop by here sooner or later.
He can wait.
He can hear his father. "We are Uchiha. We take care of our own. Uchiha come before all."
The Hyuuga are the same. The youngest generation of Hyuuga doesn't seem to know the coda, though. He blames the head family for it, when he thinks about it. He usually doesn't.
"The Uchiha come before Konoha. The Uchiha come before you. The Uchiha come before your brother. The Uchiha come before me."
The family comes first. That's the most important thing. Konoha would never get involved with his brother. He has to do it by himself.
It's why he left Konoha. His brother, despite all his crimes, is still Uchiha. He's Uchiha too. That's why he has to take care of his brother. Put him down.
He hates his brother. He loves Itachi too, and even if he didn't hate his older brother, he would still have to kill him. The Uchiha are more important than his desire. He's not going to kill his brother entirely because he hates Itachi. He's going to kill him because he loves his brother. But he hates the man too.
It's his duty to.
He likes the word duty. It's everything that gives him purpose. It makes him remember that even though he is one of only two Uchiha, he's still nothing. The Uchiha are something. Even when his brother is dead and he is the only Uchiha left, he still won't be more than one name in the list of thousands that is the Uchiha. That knowledge is why he will kill his brother.
His brother's confused. He thinks that he's the salvation of the Uchiha, or some such rubbish. One person trying to save an entire clan by himself, by killing the clan. No. That's why he will lose. Itachi doesn't have the purpose and duty that Sasuke has. The duty gives him strength. More strength than any of his friends—though there aren't any more of those now—could ever have offered.
The curse seal still burns him. He likes it that way. He couldn't tell it was there when Kakashi's seal was in place. He'd forgotten then that he'd left himself open for Orochimaru's attack. He'd been injured by someone not even as strong as his brother. The burn of the curse seal reminds him that he is not invincible. It'll be useful knowledge, when he kills Itachi. And of course, with Kakashi's seal gone, he can use the curse seal at will, if he needs to.
He feels Itachi approach. Neither of them are being subtle. Itachi because he doesn't believe Sasuke can kill him yet, maybe ever. Sasuke because he's afflicted with a ridiculous sense of honor. He's a ninja. He has duty. He has no need of honor. Itachi has no honor. The battle is a little more even than Sasuke would wish because of it.
He's ready for Itachi when his brother finally comes into view. His duty is all he thinks of as he attacks.