Manwë stumbles

[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.

[Inziladun]

Manwe was of lesser power than Melkor, wasn't he? Weren't they all?

I might take issue with that, as it is said

Manwë and Melkor were brethren in the thought of Ilúvatar.

And again

Great might was given to (Melkor) by Ilúvatar and he was coëval with Manwë.

Manwë had no concept of evil until he saw the deeds of Melkor, and even then

Manwë was free from evil and could not comprehend it, and he knew that in the beginning, in the thought of Ilúvatar, Melkor had been even as he.

I don't think there's really an answer as to why Melkor is the only one of the Valar who turned to evil. As was alluded to in an earlier post, some people are bad, some good. Free will is certainly a factor, but I think all are born with an innate tendancy toward one side of the other, to be embraced or rejected as chosen by the individual.
I think it's interesting that another Vala, Aulë, went so far as to try to make his own race of beings without the consent of Ilúvatar, just as Melkor wished to do. The motives of both were very different, and when confronted by Ilúvater, Aulë understood the enormity of his error and how far beyond his authority he had indeed gone.

I just wonder what was so special about the rebellion of Numenor that made Eru jump in.

As to that Tolkein said in a letter

Faced by this rebellion of appalling folly and blasphemy, and also real peril (since the Númenóreans directed by Sauron could have wrought ruin in Valinor itself) the Valar lay down their delegated power and appeal to God, and receive the power and permission to deal with the situation.

(emphasis added)

[Morthoron]

Melkor wasn't really bad at first, he just couldn't harmonize. He had the misfortune of being in a celestial choir with a voice like Bob Dylan (and anyone who has heard Dylan trying to harmonize will see my point).

[alatar]

Reverse that: Manwe wasn't always good at first...just doesn't work.