Eru letting Melkor go..

[Mandos]

Humans were created for one purpose and one purpose alone, to praise out God. In this life we are given a Great Gift and a Choice. To follow God or make it the best we can on our own and fall into Darkness. Tolkien knew this, this is his own hidden and at times not so sublty camolfouged, way of saying that Eru knew what would happen, but without letting Melkor go into Arda, the world would not be as it is. Just as Satan thought he could do a better job of being God and was cast out of Heaven " like a thunderbolt". God does not want anyone to HAVE to follow Him. He gives us a Choice. For there to be a Choice, there must be two sides, God's Side, and the devil's. So he must be allowed to come for there to exist a Choice. In this life we have a choice , in the next, we will be Judged for our Descision.