I guess we all knew this was coming right? How many times can a man look into the abyss before falling in? He could have become an angry, vengeful god immediately after the time war, but he didn't, because he found Rose. She, as he so simply and beautifully put it, "made him better."
It almost happened after he lost Rose, but Martha was there to temper that rage back into a manageable depression. She saved him "so many times."
The whole debacle with the master certainly could have put him over the edge, but series four gave him back Donna, who literally slapped the megalomania right out of him. As she said, she was there to "stop him."
But in the Waters of Mars, there isn't anyone to stop him. He's been alone so very long. He's let all of the pain, guilt and loneliness bounce around inside his head with nothing else to drown out the noise. So with destructive fire all around him, so reminiscent of Gallifrey and all he's lost along the way, he didn't just fall into the abyss, he threw the universe an "up yours" and gleefully walked into it. He has become the angry god he was always just a hairsbreadth away from becoming.
He's also either hallucinating or having malignant premonitions, which is never a good sign. I am worried for him and what he is going to do next.
I'm also morbidly excited to see what happens, sort of like a car accident you can't look away from. How far down does this abysmal rabbit hole go for the good Doctor? Who will be there to pull him out? Only Time will tell.