I don't recall Blizzard saying whether he was the final boss or not. They have said that we will remove him as the Horde Warchief in SoO and that SoS is the final content patch of MoP. Anything else is just player speculation.
---------- Post added 2013-02-16 at 03:07 PM ----------
I don't think he is being controlled or corrupted by anything. I think all of his emotions are being empowered by the Sha making him more powerful and more dangerous just by being on Pandaria. I do believe hi is the final boss of SoO and that we will not be killing a Sha in Org. I think they are tied to Pandaria because of Y'shaarj.
---------- Post added 2013-02-16 at 03:15 PM ----------
Except Blizz has already said he is not corrupted/possessed.