[Formendacil]

Why don't some of the Valar go bad, eh?

I don't know, but my instinct in the matter is to say that it's a done deal for them, harking back to the Ainulindalë. After all, with the exception of Men, this is "as fate" to all others, including the Valar and Maiar. Exactly how this works for the Elves, who seem to have at least some element of freedom, I've never quite been able to fathom, but that's a topic for another thread perhaps. The question here in the Valar.

With the Valar, I have no such infathomabilities, since, as I see it, from the Valar's perspective, the unfolding of history has already happened before--namely in the Music. And since they had their freedom during the Music, and made choices accordingly, to sing with Melkor's discord or to follow Eru's themes, they are bound by these choices in Arda. Their actions in history are the time-bound unfolding of their musical choices during the Music.

At least, that's sort of always been my premise. Not sure, though, that it actually bears up under full inspection.