Disclaimer: I don't own Naruto.
Hate is a blade, fashioned the way one would forge steel. It must be folded over, in airtight layers, because if that doesn't happen, pockets of air will form and allow the hate to seep out from the blade.
It took only one night for Itachi to fashion Sasuke's hate into a blade.
Threats, the acquaintance of Sasuke with the more insidious truths of the world, the sight of bloodshed and a step into the realm of the Tsukuyomi were all he needed. It took some time, but once Sasuke had managed to sort things out, Itachi knew his little brother would hate him forever.
He needed to do it. Sasuke would not have been able to go on if he hadn't had something to live for, someone to blame. Because if he didn't have those things, he would collapse, completely, as Itachi had so many years before, if for different reasons. Itachi knew that this one goal, this consuming hatred, though poisonous, would keep Sasuke alive as long as it needed to.
He didn't want to do it. He didn't want to make his little brother hate him. To Itachi, Sasuke was everything. Sasuke is everything to him. It's not a question about loving him with half of his heart or a quarter, he loves his brother with all of his heart. The rest of the world could burn, and Itachi wouldn't care as long as his brother was still alive.
Itachi hadn't wanted this for Sasuke. He hadn't wanted to expose him to death so soon (because Madara was right about something; war is hell), he didn't want to kill their parents and cut off Sasuke's heart from the source, and he certainly didn't want to leave his precious baby brother all alone.
That night was hell on earth not just for Sasuke, but for Itachi as well. It hurt to kill his parents. It hurt to kill Fugaku and Mikoto, but it didn't cost Itachi a thing to let Sasuke live. Once he had laid the blade to the first Uchiha to die by his hand that night, Itachi knew he didn't have anything left to lose. His life was forfeit; what was the cost of a small act of mercy?
But the question was, what to do? What to do, what to do? Why kill dozens and spare one? Itachi would have liked to simply kill the lot and not have to face his little brother afterwards (because he couldn't stand to think of little Sasuke's innocent eyes, cheerful and adoring), and he would have liked to be able to tell Sasuke the truth.
But neither of those situations was feasible. The latter would have filled Sasuke with hatred against Konoha (and Itachi couldn't let that happen, because it was supposed to be him Sasuke hated and not the village that, despite it's flaws, was not such a bad place), and the former would have left Sasuke confused and without purpose.
So the inevitable conclusion was reached. Itachi knew how unbearable life was if life had no purpose. Sasuke needed some sort of reason to go on living, even if that reason was hatred of Itachi. Itachi needed Sasuke to become stronger.
Because in the end, more than anything, Itachi needed Sasuke to kill him.
He couldn't say that there was no other choice, because the only time anyone said that was to assuage themselves in the decision that they had made. It was worth it, he hoped.
He didn't want to. But in the end, he had to.
So Itachi killed his emotions and shattered his soul, everything dying save sorrow, and fashioned his brother's hate in the shape of a blade, a blade that could pierce the rotten shell that was once Itachi's heart.
He just prayed that when his little brother finally came to kill him, that Sasuke's eyes wouldn't be as empty as his were.
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