Trust the idealistic idiot who never went back on his promises to fail miserably at keeping the last one. The one that probably mattered the most, in the end. The one that Sasuke had (secretly) thought he'd be okay with. More than okay, even.
Because Naruto had struck a chord when he said that he wouldn't be a Jinchuuriki and Sasuke wouldn't be an Uchiha when they ended each other, that they would be free of all the burdens they'd been carrying for years.
Ever since Itachi's death (murder, suicide, whatever it was), Sasuke was getting more and more exhausted with each passing day. The desire to take revenge was sucking what was left of his soul (after the massacre, the Valley of the End, Orochimaru, after Itachi) dry, and that burning feeling inside wasn't enough to give him the strength to wake up in the morning anymore (so he never slept when he didn't have to; he was used to running away from anything that threatened to break his resolution, that stood on his way to revenge).
What really kept him going was, pathetically enough, the fact that he had no idea what else to do. He had been an avenger for years; he couldn't remember what it was like not to be one. That word had defined him the night Itachi slaughtered the Uchiha clan and told Sasuke to hate him. It had forced him to grow up and had shaped his identity, leaving little resemblance to the small boy he once had been; he never craved affection or praise anymore (didn't have anyone left who could love him, praise him), he was only focused in destruction (didn't have anything to build a life on).
Uchiha Sasuke was an avenger, and he would always be one. But with every step he took towards his goal, he was destroying a part of himself and the thought (the truth, the hope) that it would end in their final battle had been reassuring.
Except, the outcome was everything but reassuring.
Naruto shouldered Sasuke's hatred as he had promised, but he didn't bring him down with himself. He could have, really could have, but he chose not to.
And Sasuke has no idea why. That time when they read each other's mind through blows, he'd seen the same thing as Naruto. He was sure that they would die together, regardless of what he had said to his face. He had wanted Itachi's eyes, because he could see Naruto was his equal now; their battle was going to be magnanimous and there wasn't going to be a winner.
But that idiot changed his mind at the last second and didn't use his chance. Just one second, and he was dead while Sasuke kept breathing.
Sasuke has no idea why.
He thought about it thousands of times, but still couldn't manage to find an answer. He still thinks about it, asks himself questions that do not seem to end.
Was that an act of kindness? It couldn't be. While Naruto was completely oblivious and naïve in certain ways, he was also aware that leaving someone alive was not necessarily a kind gesture. He knew that if Sasuke stayed alive, it would only be a torture.
Was that an act of cruelty? That's not even something Sasuke will (can) consider. Naruto didn't have any cruelty in his heart and he would never hurt the people he loved intentionally. He would never spare Sasuke's life to make him suffer. (That just wasn't his way of ninja.)
So why did he leave Sasuke alive? Why?
What hurts Sasuke the most, even more than the unanswered questions Naruto left behind, is the fact that this is almost a mirror image of everything that happened with Itachi.
They were both the only people he had for a while. Itachi, when his parents neglected him and Naruto, when he was too lost in his own darkness to even notice anyone else. They both understood him like nobody else did (could) and made everything more bearable. As long as he had Itachi's love, his parents' negligence hadn't hurt Sasuke as much. And as long as he knew Naruto would always be there, would always chase him (love him) no matter what he did, he hadn't felt as lonely (weak).
They both sacrificed themselves for the sake of Konoha (for Sasuke).
And Sasuke killed them both.
His history repeated itself the worst way imaginable and once again he was left with blood on his hands and the question "why?" on his lips (Why didn't you kill me? Why did you leave me alive? Why?) This time, he knows he can never even find an answer. Naruto is dead after all, and he was the only one who knows.
And this time, there is no one to take revenge from but himself. It was all his doing, all his stupidity and blindness. Naruto was dead because of Sasuke and Sasuke only.
Who was he going to kill now? Himself?
Killing himself would not be taking revenge. It would be salvation .
So, he will just keep on living (hurting) and asking questions (never to find an answer). He will always hear "Forgive me, Sasuke. Maybe next time." along with "I will shoulder your hatred and die with you." (broken promises for which he lost the right to get angry). He will bring his hand to his forehead, remembering both Itachi's touch and the feel of his forehead protector . He will recall training with Itachi and watching the sky quietly with Naruto. The piggyback rides and the sparring. The warmth and comfort.
He will take revenge from himself for everything he's done.
He is an avenger after all.