I do not think Eru wanted a bad, negative, or 'evil' force in the world, nor did he place Morgoth there hoping he would be an 'adversary.'
This is a question that comes up from time to time. Eru spoke to Melkor on his 'misbehavior' in
The Silmarillion:
Quote:
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And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument, in the devising of things more wonderful, which he himself hath not imagined.
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Melkor is not 'corrupting' the music - what Eru is saying there is that Melkor cannot corrupt the music, nor can anyone else. Freewill is part of the music. I'd definitely call him a willing participant in the music - perhaps the most eager to participate because of his desire to sing out over the others. Do as he may, Melkor cannot prevent good from prevailing. He has the choice and right to choose 'good' or 'evil.' However, no matter how many things he destroys, no matter how many people he corrupts, he cannot remove the ultimate good present in Ea and everything in it. That is, he can choose his own fate, and influence the fate of others (though it is ultimately their own doing). What he cannot do is 'create' a new 'fate' for himself that goes against Eru's creation, against the music...that is impossible. It's what he set out to do, and obviously failed.