The Silmarillion:Ainulindalë

[Rían]

by Sister Golden Hair:
But his rebellion was unproductive to the intent of Iluvatar.

Originally posted by MasterMothra

it only appeared to be unproductive. what morgoth did or did not do was all for the greater will of eru. morgoth and sauron may have thought what they did was counter-productive, but in the end they were both just instruments of eru.

"for he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined."

thats a good passage.

I was re-reading this thread and this section reminded me of something that I just read today in C. S. Lewis' The Problem of Pain:

In the fallen and partially redeemed universe we may distinguish (1) the simple good descending from God, (2) the simple evil produced by rebellious creatures, and (3) the exploitation of that evil by God for His redemptive purpose, which produces (4) the complex good to which accepted suffering and repented sin contribute. Now the fact that God can make complex good out of simple evil does not excuse - though by mercy it may save - those who do the simple evil. .... A merciful man aims at his neighbour's good and so does "God's will", consciously co-operating with 'the simple good'. A cruel man oppresses his neighbour, and so does simple evil. But in doing such evil, he is used by God, without his own knowledge or consent, to produce the complex good - so that the first man serves God as a son, and the second as a tool. For you will certainly carry out God's purpose, however you act, but it makes a difference to you whether you serve like Judas or like John.

I've seen some people say that Melkor's evil acts were a good thing because Eru produced some good things in response to them. I think that that idea is a misconception. Melkor's actions were indeed evil - but because Eru is greater, he can "trump" those actions to produce what Lewis calls a "complex good".

The question about whether Arda Remade would be better than Arda if it were unmarred - well, I don't think that Eru would be sitting around thinking "oh shoot, why can't someone just hurry up and rebel against me so that I can do a complex good!!" I think that both would be equally good, but perhaps just different - as two pictures by Monet can be equally brilliant but also different. It's certainly a complicated question.