I'd guessed for some time already that Garrosh had been corrupted. He was always an angry fellow. He hated his father for the shame he brought to the Horde by being the first to drink Mannoroth's blood, then when Thrall told him how his father had redeemed himself before dying, he suddenly loved the old Orc.
Since he came into power, he's been obsessed with the old Horde - the Orcish Horde. I've felt he was a wrong fit for leader for some time, and I'm disappointed in Thrall for leaving him in charge. Sure he could fire the people up, but he was a fool. I've felt for a long time that he's been trying to move the Horde backwards, focusing on old enemies (Alliance) rather than any true threat, and has all but alienated himself from the other Horde leaders. The Horde has been weakened because of it.
I'm not necessarily pro-Thrall - he was more of a human than an Orc - but he had the sense to see that if the Orcs didn't team up with other races, they'd be wiped out. Orcs were never a war-hungry race. Sargeras is responsible for that. Which is funny because, despite the fact that Thrall was an 'impure' Orc and Garrosh was 'untainted', Garrosh is pretty messed up.
I agree that Vol'jin should become leader of the Horde, if it isn't an Orc, because he's the oldest (ad most trustworthy) leader in the Horde at the moment. I don't know that the Tauren or the Trolls would fall in behind Lor'themar or Sylvanas. And I'm not sure they'd care for the job. Also, Baine is too inexperienced to be much use, either. It almost has to be Vol'jin, since I believe Thrall will remain a shaman.
This is just speculation. I've not read Jaina yet and Vol'jin isn't out until next month. I also don't raid, so I've missed out on any story that's there.
If someone could fill the blanks in for me and let me know just what is happening with the leaders at the moment, I'd be greatful ^^
Further speculation has it that he's not corrupted by an Old God. This is entirely possible. It's not as if evil can't exist without them - evil exists in our own world without Old Gods. I've read that some people now believe that he iscorrupted because of a datamined ability that merely contains the word 'corruption' - Warlocks have that spell. They're not corrupted by old gods.
Koruth and I believe that the Heart that they've found, belonging to Y'shaarj, is controlling the Sha, and the Sha have affected Garrosh because he's too proud and aggressive. It's not necessarily the Old God's intentions to corrupt this Orc. In fact, the Sha may well be parts of an Old God itself.
Like I've said, I don't fully know what's going on, so this is speculation on my part, as well as other people's. I can say that as soon as Garrosh is brought down, I'll be watching the video. I want to see how this goes!
On a side note, Moon Moon has gotten out of control.