Just read.... you will be amazed.

This theory stems back to what Naruto's parents said recently in the manga.  Minato said to Kushina that if he does not seal the Kyuubi into a living host, then the Kyuubi is going to revive.  If a Jinchuuriki were to die with a tailed beast sealed inside them, then at some point, the bijuu is going to revive.  Kushina stated that fact of the Narutoverse when she said that the beasts will return to balance things.  So, when Minato sealed half of the "good" Kyuubi chakra inside of Naruto, and sealed the other "bad" half inside himself, it was inevitable since that event 16 years ago, that the evil half of Kyuubi would eventually revive somewhere in the world.

But the question now is, how does a bijuu revive?  How is a bijuu born?  And where would the bijuu revive?  Especially since it's only half of the soul, what would become of Kyuubi?

So, I was thinking, the reason the Uchiha Massacre happened is because if the Kyuubi were to revive, it would be born inside an Uchiha.  That's what makes the clan "cursed."  That's why Itachi was going off rambling about "testing" his "capacity."  That's also why Shisui vanished.  Itachi DID kill Shisui, to get mangekyou, so that if the Kyuubi were to revive inside of him, he could try to control it.  That's also why Madara had an interest in Itachi to begin with... Madara thought that Itachi had high potential to be the next container.

And the Uchiha Massacre happened because the elders knew that the revival of the Kyuubi would happen within an Uchiha.  So, they sent Itachi to do the job.  As Madara said in the manga, Itachi wanted to make sure the Kyuubi's revival would never happen, since he had seen the tragic battle from his childhood.  The original intention was to kill everyone in the village, including Sasuke, so that he could, guaranteed, 100% be the container of the reborn Kyuubi, but what he wasn't counting on was his hesitation when it came to Sasuke.  The original plan was to be the sole Uchiha survivor and get the Kyuubi to revive inside of him, but the minute that he decided to let Sasuke live, Sasuke was then destined to be the next container for the bijuu.

That's why Itachi told Sasuke to live, and hate Itachi to the point that he would train to defeat him.  If Sasuke were able to defeat Itachi, then he would have the ability to properly achieve the "capacity" needed to control and maintain the beast.  But remember, Itachi didn't just leave him with that.  Itachi told him to get the MANGEKYOU.  Why would he tell his brother, so young, about the mangekyou?  It's because Itachi wanted Sasuke to be able to control the Kyuubi.  Not NARUTO, but himself!  That's why Itachi told Sasuke the directions to the tatami mat that would lead to the secret of the Uchiha clan.  That's why he informed Sasuke about the Uchiha's ability to control the Kyuubi.  It had nothing to do with Naruto, but being able to save himself.

That's also why Itachi never took Sasuke with him when he left.  He knew that if Madara got his hands on Sasuke, Sasuke would never be able to improve his skills to fight both him and Kyuubi.  Itachi spent his life trying to keep Madara AWAY from Sasuke, to protect him from Madara's influential clutches.  Also, Sasuke -needed- to hate Itachi, to kill Itachi, so he could gain the Mangekyou to control the Kyuubi... but not just that!   Itachi was NOT LYING when he said that he wanted Sasuke's eyes!  Because if he DID kill Sasuke, he would take Sasuke's eyes for his OWN eternal mangekyou.  If Sasuke died, then Itachi would be the 100% guaranteed host of the next Kyuubi revival.  So no matter what would happen, if Sasuke died, Itachi would be the next Kyuubi container and use Sasuke's eyes, or if Itachi died, then Sasuke would be the next Kyuubi, and would use Itachi's eyes.  Either way would work for them.  Both of them had the high chance of being the next container.  Both of them needed the high "capacity" for it.

Madara let Sasuke live because he saw more potential in Sasuke.  He knew that Sasuke would eventually, through his own path, end up becoming more powerful than Itachi.  And if he could manipulate Sasuke, then he'd have an even more powerful demon container to control.  Itachi knew that Madara would try to claim Sasuke after he died, so he put an auto-attack on Madara's visible sharingan.  He wanted no chance of Sasuke looking into Madara's sharingan eyes and being influenced against his control.  Madara had to start looking into methods of persuasion to control Sasuke instead of simply trying to use him off the bat.  So, he told Sasuke a PART of the truth behind the Uchiha massacre.  Yes, it's all true that there was going to be a coup d'etat.  It's all true that Itachi was a double agent.  It's all true that the village ordered the assassination of the Uchiha clan, but it wasn't because of the coup d'etat that EVERY WOMAN AND CHILD were killed.  No, it's because the truth is that they wanted to wipe out all possible containers for the bijuu!  But, if Madara TOLD Sasuke that, then Sasuke would know the truth about the demon manifesting itself inside of him.  Madara wanted to keep Sasuke in the dark, because he wanted Sasuke to continue to improve himself for the sake of revenge, in the name of anger and hatred.  Because, the more anger and hatred Sasuke had, the more the demon would manifest itself within him.

When Karin looked into Sasuke's chakra and saw a demon manifesting, the most evil thing she'd ever seen in her life, it's because that's the "evil" half of Kyuubi, and it's taking over Sasuke's entire personality.  The more he continues to wreak his revenge, the deeper he's falling into the abyss of anger and hatred, and losing a part of himself to the demon that's reviving.   When Karin saw Naruto for the first time and her eyes went wide in shock at seeing his chakra... She saw that it was the SAME chakra inside of Naruto, and didn't understand why.  Naruto had the other half contained within himself.

And when Naruto said to Sasuke that if they were to fight, they would both die, it's true in a way.  Since they're both carrying the opposite halves of the Kyuubi's chakra, they're linked by a thread of fate.  It's very likely that their power is going to leave both of them equal in strength against each other.  The end result is going to be a stalemate unless some other factor jumps in to assist them.

Sasuke's chakra containing ability is also increasing.  After all, he was winded after one battle against Killerbee, but then later, he took on all the samurai, all the kages, Danzou, AND Naruto, and was still standing to tell the tale.  His chakra has increased tenfold, and as the manga progresses, it will continue to increase, and he's going to fall further and further into darkness.

But it also means that Naruto is the only person who can save him.  As the other half of the chakra, the "good" half versus the "evil" half, he has the ability to relate to Sasuke and communicate with him.  His words have more ability to reach him than anyone else's.

The part that I can't quite predict is how the manga will end if all of this theory I have is true.  The world can't be balanced with both halves co-existing inside of Naruto and Sasuke.  Madara also has his plans to extract the Kyuubi.   But remember, Madara also said that Sasuke will be the one to fight Naruto.  Perhaps he's hoping that if Sasuke manages to kill Naruto, then Sasuke will fully revive as the Nine Tails.  For the plot to truly progress for us to see that, Naruto would then have to die (and/or have the Kyuubi extracted, which would also lead to eventual death).  But, since he's covered by plot (and manga title) protection, that's going to be a tricky one... especially since the last person who knew how to soul transfer died when she revived Gaara.  

In any case, as for my theory regarding the truth about the Uchiha Massacre and Sasuke being the next container, the legitimate revival of the Nine Tails instead of being a jinchuuriki... I was wondering if any of you had any thoughts on it.  It really makes a whole ton of sense to me.