The real question I have is this If the Juubi has always been evil or mindless, it would have destroyed all people long before they had a chance to form clans or communities. How is it that the human race survived long enough for the Sage to be born and grow into adulthood? Wouldn't the Juubi have destroyed everybody before that?
This gives us two options:
1) The Juubi didn't always exist (or did exist, but was trapped somewhere)
2) The juubi wasn't always a threat to humans, but turned into a threat at some point.
Other questions we can ask are these: Why does the Juubi have an eye that looks like a Rinnegan? Is the Juubi eye related to the So6P's eye? If so, are the Juubi and the So6P related somehow? If so, and if the Juubi powers still exist in humans (Kurama's powers clearly affect Senju wood release, and Uchiha eyes clearly can still turn into Rinnegans), is the Juubi somehow related to all of mankind?
I've got a few off-the-wall theories related to this, but nothing that's worth talking about now. It's quite an interesting thing and I really hope that Kishi gives us more information about the Sage and the Juubi, and the great fight they had with each other.