The glimpse of Melkor and Eru's dynamic we get in the Ainulindalë is really interesting. Eru is not angry in the beginning of Melkor's discord. He takes it with a smile on his face, and that makes more sense with the information that Melkor 'was to make I devise I begin', as we learn in Morgoth's Ring.
Then Ilúvatar arose, and the Ainur perceived that he smiled; and he lifted up his left hand, and a new theme began amid the storm, like and yet unlike to the former theme, and it gathered power and had new beauty. But the discord of Melkor rose in uproar and contended with it, and again there was a war of sound more violent than before, until many of the Ainur were dismayed and sang no longer, and Melkor had the mastery.
Eru even alters his initial theme, but Melkor still tries to contest him and to drown him out:
Then again Ilúvatar arose, and the Ainur perceived that his countenance was stern; and he lifted up his right hand, and behold! a third theme grew amid the confusion, and it was unlike the others. For it seemed at first soft and sweet, a mere rippling of gentle sounds in delicate melodies; but it could not be quenched, and it took to itself power and profundity. And it seemed at last that there were two musics progressing at one time before the seat of Ilúvatar, and they were utterly at variance. The one was deep and wide and beautiful, but slow and blended with an immeasurable sorrow, from which its beauty chiefly came. The other had now achieved a unity of its own; but it was loud, and vain, and endlessly repeated; and it had little harmony, but rather a clamorous unison as of many trumpets braying upon a few notes. And it essayed to drown the other music by the violence of its voice, but it seemed that its most triumphant notes were taken by the other and woven into its own solemn pattern.
It seems to me that the third theme, which was the one that created the Children, was a musical manifestation of the natural progression of the music of the Ainur. The more I read this passage, the more it seems that Eru tried to incorporate Melkor's music into his own. And this is fascinating to me because it shows how essential Melkor was in Eru's plan. Eru did not want to exclude him. He did not want to suffocate him or for him to be powerless. But he DID need him to be part of a whole. It is not Melkor's original ideas, but his desire for dominion that angers Eru.
And yet I am not Eru's fan because I DO think that when he addressed Melkor after the discord he said the worst thing that anyone could say to someone like him. "[...]this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined' was a savage thing to say to anyone and it's one of the cases where Melkor's feelings seem reasonable to me. I don't think that anyone would have been ok with being told they are a tool in the hands of someone with better ideas than their own. Iluvatar is like a lousy father who loves his child but cannot express himself properly and only ends up pushing said child away from him.
