Disclaimer: I don't Naruto.
When Sasuke arrives late to the chuunin exam, Hanabi wants to throw up at the sight of him. It's not because of his arrogance because arrogance is like breathing to a Hyuuga. There's something niggling at the back of her mind and she wants to shy away and she wants to kill him for making her want to shy away but most of all she wants to scratch, tear, rip her skin off because it, he, feels wrong.
She doesn't understand the loathing nausea he induces until long after the leaves and dust have settled and he is standing sideways on a wall.
She doesn't understand it but she can taste it. A bitter, acrid taste that reminds her of rotting corpses. The shadow of something else, something wrong inside him, creeping and slithering through his veins, hiding in his heart and devouring his body, cell by single cell
Hanabi knows that everyone has a darkness inside them but she can see Sasuke and she can feel him somewhere in her head, lingering just next to the spot she thinks of as her Byakugan. And what she can see in that secret place in her mind is him and his darkness and someone else's darkness hiding behind the first, a black and purple, writhing, thing inside the Uchiha that isn't his, isn't him.
Until it is.
Because once its crept up through his blood and into his brain, suddenly the darkness will be his and will be him and he is the darkness and the darkness is him and it won't be wrong anymore. Instead he is. Instead it's his chakra that fighting to survive, struggling to reclaim its territory and drowning in the hate and anger and pain that shouldn't be in the Uchiha but somehow is and it's Sasuke's chakra that's becoming an abnormality in his own body.
Hanabi closes her eyes to block the growing, throbbing, mass of purple-black cancer and knows with chilling certainty that no matter what happens, the Uchiha clan will soon be down to one.
oo0O0oo
In the Valley of the End, Sasuke kneels over Naruto, his hair drips cold drops of water onto the unconcious boy. He is so close that he can hear Naruto's chest gurgling with blood and if he leans just a little bit further, his nose would be able to touch the strange, faint whisker marks on his cheek. His panting breaths brush over Naruto's face, stirring blonde locks plastered to his head by the water. As he struggles to his feet and begins walking towards his destiny and his death, he wonders why it's harder to push the cursed seal back than it is to call it forth.