If Sasuke were to kill Naruto... Oh, dear. A lot would happen, really, for everyone. And people, like others have said before me: just because Naruto is the main character doesn't mean that he can't die. Now, do I think he will? Not really. Do I think that, if he does, he'll be revived? Yes. But for this thread's question... If Sasuke killed Naruto, I think...
1) The entire fanbase would explode, either with the intense feelings of sadness and rage from Naruto fans or the happy cheers from Sasuke lovers/Naruto haters. It might even manage to start flame wars, somehow.
2) Jiraiya's dream of peace would shatter, the last fifteen years since Kishimoto first published Naruto in Japan would've been for nothing, and it'd have all been just pointless. This is why I doubt it'll happen; I can't see a future for the Shinobi World without Uzumaki Naruto. I guess I'll say the Allies would have an even stronger reason to fight, especially his friends.
And 3) Sasuke himself. Would. Break. No, no, I really think this. I don't mean in some sappy, heartbroken way. But upon Naruto's death I'm sure he'd have some sort of serious revelation, be it from Plot no jutsu, a last minute speech from Naruto or, what I think it'd be, their past. Because truthfully? You will never convince me Uchiha Sasuke hates Naruto. You'll never convince me he wants to kill Naruto out of hatred towards the Leaf. No, no. I don't see it that way. It's just my opinion, but to be frank, I see Naruto as an obstacle for Sasuke; one he can't just conquer because Naruto is an emotional obstacle for him. Hence why I think he's so eager in the manga right now to get to where Naruto is to kill him; to cut down his last living weakness in the world. To truly be free to destroy the Leaf... Taking down Naruto would be a bonus on that end for him, of course, but I still feel like if he ever did he'd immediately realize he made a grave mistake. My two cents. Or, well, let's call it a dime.
Last: Do I think they'll die together? Not really. Again, can't forsee a future without Naruto. Do I think Sasuke will die? Maybe, but only in a self-sacrifical way, it seems. Let's have a redo of the Wave Country Arc or some shizz.