I haven't actually seen the episode yet, but wasn't his becoming the Valeyard already prophesied in "The Name of the Doctor" ?
Yes, yes it was, from what I recall.
From what I understand, the Doctor himself won't actually become the Valeyard; the Valeyard is more like a distillation of all the dark aspects of his character, with a somewhat tenuous existence. Perhaps he's some kind of corporealized psychic projection, like the Watcher was for the Fourth Doctor, or like that Time Lord-turned-Tibetan monk had at the end of the Third Doctor's run?
cheers,
Gabe
Rule 1: The Doctor lies. The Master, more so. :P (Only kidding.)
On a more serious note, I wonder if the Valeyard, on top of being a dark distillation, might've wanted to change the Doctor's history in order to a)avert the (potential) "destruction" of Gallifrey, or b) go ahead and destroy Gallifrey with the Moment.
(OK, tonight's not a good night to drink vodka. :P Still, though, before I saw the 50th anniversary episode, I actually wondered if the Valeyard might've been trying to manipulate things so that he could keep himself from destroying Gallifrey. Course, after watching the 50th, I see that that might not have been the case, but meh. Like I said, don't ever get me posting when I've had a wee bit too much to drink.)