In the Ainulindalë Melkor chooses to follow his own will rather than the design of Ilúvatar and to usurp the act of creation (S 16-17). It's noteworthy here that in response Ilúvatar does not cast Melkor out, but tries to explain to him why he is unable to create independently and the harm that his attempt to do so has caused for the created world. Ilúvatar even allows Melkor to go down into the world on the pretext (advanced by Melkor) that he will work to repair the harm he has done...
