Quote Originally Posted by Super5
2) Itachi definitely has some purpose in killing his clan other than "testing his container/ability/whatever". That is an incredibly lame excuse and doesn't fit with Kishi's expository style. I don't think his purpose has anything to do with having 3 MS users, but it definitely involves the Tengu king and the Uchiha past since Itachi pointed Sasuke to the shrine after he let Sasuke live.
There's obviously a lot of backstory regarding Itachi killing the Uchiha clan. But I don't think it's necessarily incompatible with Itachi wanting to find out how strong he really is. Keep in mind that in traditional Japanese stories, the Tengu are very ammoral characters. Sometimes they're good. Sometimes they're really bad. But they're always egotistical. They're also pretty much always strong enough to get their way. Sounds a lot like Itachi to me.

In any event, there are a lot of problems with the whole theory as presented on the other board. For one, if Amaterasu's black fire requires Kyuubi chakra, Itachi wouldn't have been able to make it. Kyuubi had been sealed inside of Naruto for like 13 years before Itachi, Sasuke, Jiraiya, and Naruto met at the hotel.

Similarly, Madara wouldn't be able to use Kyuubi's chakra unless Kyuubi was sealed inside him. If he was, Kyuubi must have been unsealed at some point, or Kyuubi would be dead. (Recall the conversation Naruto and Kyuubi had in Naruto's mind)

And again, according to the theory, Madara sealed up Sojobo. But where? The smart thing would have been to seal it up inside himself, so that he could use its chakra like Naruto does with Kyuubi now. Moreover, Sojobo would have died with Madara when he died.

Okay, here's my speculation, as inspired by the thread starting speculation.

1. There's Tengu blood in the Uchiha clan.
2. Madara and Shodaime were close friends. They disagreed over something very important, and Madara killed him over it. Madara gained the MS. (I haven't read the manga, so I don't know if Shodaime's death has been discussed).
3. The MS is the only way to get some specific super secret Tengu power.
4. Itachi found the shrine, learned about he MS, and learned that with it, one can get more Tengu power.
5. Itachi killed his friend to get the MS, but was unable to get the other, super secret new powers.
6. Itachi killed the clan to get Sasuke to try to get the super secret powers. Why? Because he's bitter that he didn't become stronger and wants to prove to himself that it isn't because he just isn't capable. Or, similarly, he hopes that Sasuke can get it just so he can kill him and know not having it wasn't a big deal in the first place.

All the symbolism from the original speculation works for mine.