[Legate of Amon Lanc]

Originally Posted by Roa_Aoife

An interesting question, to be sure. I'd have to say there's a yes and no answer. Melkor, from the beginning, was the cause of the discord, and hence the evil in creation. It wasn't necessary for him to enter it for evil to exist, but then again, it WAS necessary for him to enter. Recall that Men are distinguished from both the Elves and the Valar and Maiar in that Men are not bound by fate. Melkor, as one of the Ainur, was bound. From the moment he began the discord, his fate was sealed. One could say his role as the embodiment of evil was inevitable.

I have only to second Roa, indeed, that's probably the way to see it. I would have posted similarly, now I could only say the same in other words. You could also say, from another point of view: if you look at the Ainulindalë, Arda was somewhat pre-shaped by the Music, yet when Valar came down, all was yet empty and they had to labour hard to make all the things - which already were in the Music - came to being. And at that moment Melkor came, and kept destroying what they built etc..
So, in fact, we can say that from the moment Melkor started his dischord, there really was no other option for him than to come down to Arda and keep destroying the other Valar's work, likewise the other Valar had to come down and grow plants and raise mountains etc. If Melkor chose not to come down, there will be simply a "logical flaw" in the creation. There was direct causality between the Music and the reality of Arda itself. In this, the Music was really much more for the Valar than they "thought", also because it simply predetermined their actions (now if saying "pre"determined is correct, because it was in the so-called Timeless Halls, so you could actually say that the Music took the place at the same moment as when the corresponding events took place in the world, if you get what I mean). The only one then, who could intervene into the world behind the pre-determination of the Music, would be Eru himself, and then of course his Children (who again themselves are his intervention).