Sasuke should have seen it coming. They all should have, really, but him most of all. And it burns in his belly like acid that he didn't. Beyond that, even – that he saw the signs but disregarded them, paid them no heed, enjoyed them even, not realizing the dark meaning of them.
Naruto has always been a wild person, energetic and eager in every aspect of his life, from training to recreation to sex. And Sasuke, no less eager and rather fond of rough play, curses himself that when Naruto's wildness began to grow more intense, he merely rode the wave of enjoyment, threw himself into the rough training sessions, thoroughly enjoyed their verbal spars, and abandoned himself in the intense pleasure of their nearly brutal sex.
He thinks to himself that he should have realized what was happening, should have known that the signs all pointed to one conclusion. It shames him, makes him feel ill, to think that he merely smirked to himself in satiated contentment when he looked in the mirror in the mornings, admiring the long, thin scratches – no, not scratches, cuts – that striped down his back, or the shorter, deeper ones decorating his hips. He should have realized that no human fingernails could have made those.
But he didn't realize. He knows he should have, but back then, he was living in such a fog of happiness. Naruto is one of the Leaf's foremost jounin, regularly taking on and demolishing S-rank missions with ease. His aptitude in utilizing the Kyuubi's chakra... well, that's what brought him where he is now.
Kneeling, bound, his body decked with paper seals that can't possibly hold him for long.
It was Iruka who bore the brunt of the breaking dam – Iruka who is now lucky to be alive, surviving only because of Tsunade's incredible skill as a healer. Iruka, who went to awaken a napping Naruto and found himself facing a furious Kyuubi in possession of the young man's body. Naruto himself would never, ever have attacked his beloved Iruka-sensei; the creature that stared out of those blue eyes was unequivocally not Naruto.
Gaara had said something about it once, long ago. About how Shuukaku would erode his mind as he slept. Unbeknownst to them all, the same thing has been happening to Naruto, the fierce animal instinct of the Kyuubi overtaking the human's reason and gentle nature.
"We should have guessed," Tsunade whispers, behind Sasuke. It isn't the first, or the second, time she's said it. "With how often he was using that chakra... how many tails he could summon... there was no way the seal was fully intact with that constant interplay."
The seal, once upon a time, was a dam that let through only a thin trickle of power. But as Naruto began to demand more and more of that power, to use it more and more frequently, it has eroded, until what's left is barely enough to protect the existence of Naruto's spirit. No one can tell for certain when the last vestiges of the seal will finally give way, ending the existence of Kyuubi-sealed-in-Naruto and fully realizing the situation of Naruto-possessed-by-Kyuubi. But everyone can tell it will mean disaster.
Naruto knows it best of all. The moment the Kyuubi's rage faded, the moment he reasserted control of his own body and found Iruka bloody and savaged at his feet, he realized, and he has told Sasuke that this is how it must be. His first act was to call for a healer, for Tsunade, for anyone who could save Iruka's life. His second was to turn himself over to the ANBU, confessing exactly what had happened.
And so it comes to this. Naruto is bound, with seals plastered all over him – not enough to reinforce the Yondaime's eroding seal, but enough hopefully to prevent a last act of self-preservation on the part of the Kyuubi. Jiraya and Tsunade are certain that if the strike is made before the seal is destroyed, that the Kyuubi will not survive the death of Naruto's body.
The sword that ended Itachi's life five years ago, freeing Sasuke from his self-imposed mission of revenge, will now steal away the life of his lover. It is Naruto's only dying wish: that Sasuke, not some faceless executioner, be the one to carry out the sentence.
He kneels down in front of the blond. "Naruto..."
A weak grin is his answer. "Sorry, Sasuke. I guess I shouldn't have asked you..." Naruto knows that this is torturing Sasuke. "Just think of all the times you've been really mad at me. That'll make it easier, right?"
Sasuke recoils in horror. It does no such thing; in fact, it makes him ill to think of Naruto associating their – admittedly frequent – fights with the act he's about to perform. "Naruto!" He hisses again, his chest aching.
"I really am." Naruto drops the weak attempt at cheer. "It's going to hurt you a lot more than it'll hurt me... I'm not going to be around to worry about it. Just... try and move on, okay?"
Sasuke has no intention of doing any such thing. "Idiot," he murmurs softly, blinking back tears, and leans forward to press his lips against Naruto's one last time. "Idiot. I love you."
Then he stands. Draws his sword. The edge is very sharp; he spent hours yesterday honing it. It will cut through bone like butter; Naruto won't suffer. His body aches with the memory of their last furious hours together, in bed, making love like there's no tomorrow.
There isn't.
He raises the sword, lowers it slowly until the blade just barely kisses the hairs at Naruto's nape. That's the path it will travel. Raises it again, and this time the fall is swift, a single fast movement that ends in a spray of hot blood and a sickening thump.
Sasuke's nerveless fingers release the hilt, and salt tears mingle with Naruto's blood on his cheeks.