Board Thread:Theories and Speculation/@comment-25139119-20150820032806/@comment-734582-20150923201730

BakumatsuWarrior wrote: Another question: why did Kaguya turn into the Juubi in the first place? Couldn't she have simply absorbed the Shinjuu and then battled her sons like she battled Nartard and Sasgay? The Juubi displayed little to no intelligence during the War. So why did Kaguya transformed herself into a stupid, unintelligent monster? She was already dumb to begin with. I suppose that answers the question. She was dumb enough to go even dumber. But that sounds really lame. Considering her obsession with being the only one with Chakra and returning it all, it's quite possible that the eater of the Chakra Chakra no mi is somewhat of a slave to the Shinju, or at least increasingly becomes one over time. It's possible the Shinju called out to her to merge.

Alternatively, we've seen Obito nearly loose it to the Juubi. It's possible Kaguya tried to do the same, absorbing the Shinju for power, but lost the battle (mentally unstable and all that) and became a crazy monster.