Silmarillion - Ainulindale

[Hilde Bracegirdle]

J.R.R. Tolkien
...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.

I think it might be important to note that in this quote it does not say that 'nothing maybe played that hath not its uttermost source in me', but rather 'no theme maybe played'. The themes were composed by Ilúvatar, and Melkor's improvisation cannot alter those basic themes, even though they were not in the sheet music. He cannot usurp the role of composer.

Even though Melkor is a product of Ilúvatar’s thought, this does not necessarily mean that Ilúvatar has an element of evil within him, though he undoubtedly can conceive of it. If Melkor had been a product or aspect of Illuvatar's personality, than you would indeed be right. But as it is, one can imagine evil without being evil one's self.