One missed step, and Naruto does die at the Valley of the End. In the aftermath, Sakura does what must be done.
With desperate, longing rage filling his head and water-slicked stones beneath his feet, Naruto stumbled. He was already exhausted beyond the point of his inexplicable healing abilities.
With combined fury and despair blinding him, Sasuke staggered and didn't quite shift his aim far enough. At the last minute, something inside him had said /go wide, go wide of Naruto's heart, let him live/--
--but before he had time to think about why he didn't want to kill his best friend after all, his hand was closed around the shredded remnants of Naruto's heart.
They were closer than anyone had ever been to each other in that moment, and still there was a wall between them.
"I can't believe you actually did it, you bastard," Naruto said, even though he should have been far past speaking by that point. But it was Naruto, and he had never adhered to should-have-beens.
"I didn't mean to," Sasuke said, shocked into admitting it.
"Figures," Naruto said. And then panic replaced the bewilderment in his eyes. "Sasuke! Don't let them take me back!"
"You're not in a position to care," Sasuke said.
"Don't let them take my body back," Naruto corrected himself. There were tears trickling down from the corners of his eyes in a pattern quite like the way the blood trickled from the corners of his mouth. "I--I promised Sakura that I wouldn't come back without you..."
"Why the hell would you do a thing like that?" Sasuke demanded, but of course that was when Naruto's body finally gave out.
Sasuke pulled his hand out from inside Naruto and let the body fall. He was too busy staring at his hand, red and covered with shredded muscle and pulverized bits of bone. He wondered if that was the color of his insides, or if they'd turned black by now.
The strange thing was that his heart felt like it was the one that had just been crushed.
Afterwards, still dripping with blood, Sasuke incinerated Naruto's remains. He thought of it as a last favor to a friend, but it didn't really work--no matter how hot his fire, he couldn't destroy the twisted metal of Naruto's hitai-ate. Eventually, he realized that other Leaf ninja would be coming, and he fled. He took Naruto's ashes with him, but left the hitai-ate.
He had intended to go to Orochimaru, of course, but somehow the snake's promise of power seemed hollow now. He told himself he'd still go--he still needed to hone himself, after all, temper his edge--but first, he needed some time alone.
The forests he lurked in were not very welcoming. He found himself ill at ease, starting and staring at any sign of another human being. He kept thinking that Naruto would lunge through the undergrowth, pin him down, and demand an explanation.
He was unutterably relieved when he finally managed to manifest the Mangekyou. For the first week of his hiding, he feared that it had all been in vain, that his eyes wouldn't change at all. But finally they did--and then came the unexpected.
Despite the evidence, he began to doubt.
What if the Mangekyou's appearance was a coincidence? What if it hadn't been triggered by Naruto's death, but some other aspect of the battle? What if there were a way to gain that power without Naruto's death? What if, what if--
He tried to tell himself that it was useless, that he shouldn't be worrying about the bonds he'd just broken. He was stronger now.
Still, sometimes at night he woke up in a sweat, and he'd leap out of his makeshift bed with a kunai in his hand. The rage and despair was nearly overwhelming, and he wanted desperately to just slice open his own flesh to let the hurt out. Only the discipline of all his training saved him, reminded him that such self-harming behavior would be wasteful and futile.
After a few weeks, the doubts began to go away, and his goal began to reassert itself. It was a blessed relief. He'd let himself be weak for a while, but finally, (the memory of) Itachi saved him once more, set him on the right path.
It was good that Naruto was dead. Now there was nothing stopping him. Now his path was clear.
At about that point, the fevers began.
Orochimaru had grown quite tired of waiting for him, and then of searching for him. Now he was pulling on the curse seal, trying to reel him in by pure psychic force. Sasuke found himself curled up in a little ball at night, biting at his own hands and arms, sometimes until he bled. He wasn't ready to go just yet. He needed time to begin to unlock the Mangekyou on his own.
But it was exhausting. He was nearly ready to give up and respond to the call when he woke up from one dazed fever dream to find Sakura tending to him.
She was red in his vision now, distant. Sasuke thought he might like that. It made dealing with her easier. He didn't have to worry that she'd be able to touch him in unexpected ways quite so much.
"What are you doing here?" he said once he could force his voice out from his throat. "Other than getting in my way."
She did not answer him; instead, she picked up a damp cloth and dabbed at his forehead with it.
He attempted to swat her hand away, but he was still too weak to put enough force behind the gesture. His hand fell limply on her wrist. After a moment that seemed oddly long, where her skin felt cool and soft under his palm, he pulled away. Glaring at her would have to suffice for now.
"I told you," he said. "You're annoying and you're getting in my way. I thought I got through to you back in Leaf."
"I thought you did too," she said softly.
"If you're here, I didn't! Useless girl," he added, throwing as much sting into the words as possible. But he suddenly wasn't entirely sure why he'd wanted her away from him in the first place.
She was getting in the way, he reminded himself. She made you think things, do things that an avenger shouldn't.
But she couldn't touch him that way now. Naruto's blood lay between them as a moat. He was sure of it.
Maybe it would be all right to have her along after all. She would get killed eventually, after all--
--and for a moment, his throat tightened at that thought.
But then it passed. It didn't matter anymore. He started to close his eyes.
Suddenly she grabbed the damp cloth off his forehead and clenched it tight. "Why did you do it?" she asked. There were tears in her eyes again. He hated it when she did that.
She glared at him.
Sasuke shrugged. "He was standing in the way of my goals," he said. My hand slipped, he did not say. He wondered why he wasn't celebrating. He had achieved his goals.
"I would have helped you reach those goals," she said. The tears had spilled over onto her face. It made him uncomfortable, pricked at the heart he'd been ignoring ever since he crushed Naruto's.
"You'd have been useless," he said. "You'd have gotten in my way. Like you're doing now." For a moment, he wanted to add, You'd have gotten hurt, but then the red settled over him once more, and he didn't feel the need to worry.
The red was coming more often now. It was a welcome relief from how hard it had been before, when he'd struggled with Naruto and worried about Sakura. When it was at its strongest, he wondered why he hadn't taken Itachi's advice long ago and killed Naruto then. But then, maybe Naruto wouldn't have counted as his best friend then--although he was starting to wonder why he'd considered anyone a best friend. It was a silly notion. He was a little bit glad to be rid of it.
But he looked at the tears on Sakura's face, at her eyes, and he felt maybe a little regret.
"You should rest," she said.
"I don't need to do anything you tell me to do," he said. "You're weak. I could swat you away like a bug." He started to push himself up, but he felt his arms go out, and he sank back against the ground once more.
"Not while you're still weak from fever, Sasuke-kun," she said. "Rest now."
He glared at her, but he could only keep it up for so long. He was tired. His body still ached. It was not long before sleep claimed him as its own.
And so it was over the next three days:
"You're useless and annoying. If I could stand, I would strike you down where you are." He would glare.
"But you can't, so rest now, Sasuke-kun." And she would continue mixing herbs and keeping watch.
Eventually, he began to consider taking her with him when he went to Orochimaru--/if/ he went to Orochimaru. After the fevers, he was a little disdainful of giving himself over to him. But maybe he'd submit, just for now. And Sakura's insufferable patience might be a useful tool to him in the darkness there.
Every so often, though, he'd wonder if maybe it wasn't such a good idea--not because she wouldn't be able to benefit him, but because she might get hurt, and that would be bad.
Then he wondered if that was so bad after all. She had never been anything but useless and annoying (although occasionally he would remember that he had once thought differently).
Still, even in the red, his thoughts chased each other around his head till they nearly made him sick and feverish again. Then Sakura would come to his side, tend to him, and give him medicine, and it almost felt good. Eventually, he would drop off into a deep, healing sleep. Something about Sakura's presence made him sleep more soundly.
After the third day, though, he had a strange dream.
It felt like he was half-awake, but removed, his eyes just a little bit open but his body unresponsive, as if he'd been drugged. But Sakura would not have drugged him with anything but the medicine for the fever--she was too useless and infatuated--so he knew it was only a dream.
Kakashi was there in his hidden clearing along with Sakura. The sight of the two of them together almost made him hurt. Almost.
"I'm running out of time, Sakura," he said. "It was difficult enough to get them to give you this mission in the first place."
"As far as they knew, I was the only one who could do it without a struggle," she said, but her gaze was downcast. "They didn't know he'd be feverish and wrecked when I found him, in a state where anyone could have taken him out without trickery."
"You sound glad," Kakashi observed.
"Just give me another three days, Kakashi-sensei," she said, her voice a little choked. "Please."
"Two," he said. "That's all we can spare."
"I suppose it doesn't matter," she said, and she sounded older for a moment. "He's already dead."
The dream ended then and returned to the red darkness that haunted his sleep now. He did not think much more of it.
When he woke up on the fourth day, Sasuke had only this to say to Sakura.
"Have me healed by tomorrow. We'll leave for Sound then."
"We?" For some reason, the way she looked at him was sad.
"I've decided to take you with me," he said. "You could be useful in your own way."
"All right, Sasuke-kun," she said. "But I don't think you'll be ready to leave by then."
"Make me ready to leave," he said. "Or I'll dismiss you, and you won't be able to come with me."
She bowed her head. "I'll see what I can do. Maybe this latest medicine will finally put the fevers behind you."
Something in him stirred, told him to perhaps not take the medicine this time. But he dismissed it. Before the red took him, he might have overestimated Sakura's capabilities sometimes, but now he knew that she was useless and pathetic.
Or so he told himself.
When the next morning came, he was weak and his arms hung limp at his side, but he could stand and he could walk.
"Good," he told Sakura. "You've done your job."
But she gave him an odd look. "No," she said quietly. "I haven't." And she walked over to a tree where she'd leaned her supplies when she first arrived.
"It doesn't matter," he said dismissively. "You'll come with me to Orochimaru now."
"Of course you will," he said.
She leaned down, picked something up from the pile of her things, and drew out a short, sturdy sword. "I don't think I will," she said.
He stared at her. Beneath the red in his mind, something started to warn him to run--but then it trailed off. "Sakura. If you come with me to Orochimaru, when this is all over, I will make you my wife."
But she smiled at him, and it was the strangest, saddest expression. "I don't think you mean that, Sasuke-kun. Maybe once, someday you would have asked me to be your wife and meant it. Someday, /maybe/. But now?"
--she's right, I might have, came the slightly surprised thought from beneath the red. But it was an absurd thought.
She put her hand on his shoulder, and she pushed him down. To his surprise, he felt his legs buckle beneath him, and he fell.
"I loved you," she said. She was close now, close enough to kiss him, and for a wild moment he thought she might do just that. But instead she simply said, "That's why I'm doing this. Not for vengeance, and not because the Leaf told me to."
He could do nothing but watch the flash of her steel as it cut into and then through his throat and neck.
Sasuke had heard, once, that victims of decapitation could remain conscious for several seconds after the act. It was true.
(And something about it seemed familiar somehow, made him think of putting his hand through Naruto's chest and walking away afterwards.)
The last thing he saw was her eyes. There was mercy in them.
And then, finally, in the black, he was free of the red.