I think Sasuke is a strong aspect of the series. And I think many people read some of his acts wrong. He DOES like Naruto and Sakura. That's why he tried to leave the village in the middle of the night without them knowing. If he had really planned to murder Naruto, then why leave Konoha first? He could of simply visited Naruto, waited until the knuckelhead turned his back and just stabbed him or something. But he wants Naruto to stay alive and fulfil his dream (he even says so, kinda, when they first meet in Shippuden). And even when they fight in the final valley (and let's remember that the curse mark is obviously having a strong influence on his hate and pain, like a drug) he still tries to talk Naruto out of a fight. Why doe that if he DOES want to kill Naruto? Yes, he does it in a rude manner, but only becouse it would (1) be more painful to have a friendly and kind conversation - and (2) he knows that Naruto can't be on this mission all alone (I mean, why else should the sound Ninja be missing?). And it is only when Naruto refuses to leave that Sasuke tries to swallow down his feelings and get the "Job done". And yet, he can't bring himself to really kill his comrade. In his eyes the hidden leaf failed to kill his brother - or atleast put him in Jail. The third Hokage - in theory the strongest Ninja in the village - was killed by Orochimaru. And Sasukes Teacher was defeated by Itachi as well. AND Itachi - as he says in Sasukes presence - is part of an secret, most likely evil Organisation. So time is running out. And did you notice how quickly Orochimaru arrives at the end of Sasukes first Shippuden Fight with Naruto? Doesn't that imply that he was keeping an eye on this confrontation for some time now? And woudn't Sasuke see through that? I think he speculated that Sai would stop his attack. He hates Orochimaru (even says so). And I don't think that he ever planned to give that guy his body. But in becoming Orochimarus apprentice he could do three Things: Keep an eye on that guy.
Obtain further strengh.
Take down another villain on his quest to stop Itachi from cousing more harm. Before the Kage Meeting Sasuke does all he can to never peform an unnecessary kill. Even in the training with Orochimaru. I dare to speculate that Sasuke wanted to murder Itachi in order to prevent people going through the same nightmare as him. And once that would of been done, he would of gone in to exil, find a small village and live a peaceful life as a farmer, guard or something. Or return to the leaf and explain himself. He isn't an idiot. He knows that Tobi isn't his friend. That he told the truth about Itachi, becouse he knew that Sasuke could never forget that, live on. So he could use him for his own goals. But the fact remains, that he DOES now know the truth. And let's not forget that Sasuke is now a Rogue Ninja. And Kakashi clearly states that Rogue Ninjas get killed. The only reason why Tsunade didn't order for that was becouse she knew how Naruto would feel, maybe she was able to see though Sasukes deception towards Orochimaru, hoping he would come back after this. Hoped, that Team 7 (similar to her own) coud experience a better ending. Remember when Sasuke noticed how Konoha followed him (after he "killed" Orochimaru) and he destroyed Narutos Shadow Clone? Sasuke doesn't feel worthy of love. So from his positon it makes no sense that Naruto (still) would think of him as a friend. He must think Naruto now (in the best case) only want's to bring him down. And (in the worst case) take revenge himself. It is only after Danzous death (remember that Sasuke at this point feels like his live is worthless) That Naruto makes clear to Sasuke how his intentions never changed. Sasuke want's to believe in Narutos words. But how could someone love him? And how can he feel that wish? Did he learn nothing? But Sasukes sees no way out. He probably thought the leaf chased after him ever since he left. That they want to kill him and Kakashi is lying. And now that he knows the truth about Itachi (and of course he believes it. To not believe it is to face the fact that HE killed his own brother for literaly nothing - which is to mutch to shoulder for most adults, let alone a teenager) it gives him something to live for. The war against the people he blames for everything. Who (as he believes) want to kill him anyway. And let's not forget: Even with Sasuke helping to end the war and defeat Kaguya, the decision about Sasukes fate afterwards came out very close.