Silmarillion - Ainulindale

[Macalaure]

The way I take this passage is that Eru created the phenomenon of Music, so if Melkor chose to introduce a loud strain of dissonance, Eru could still make some pleasant music result.

How I read it is, that the dissonance, though played with the intention to disturb, still has its source within Eru, and therefore is no dissonance, but only perceived as such.

I do not think that this implies that Eru is evil. It is interesting that later on the degree of evil in Sauron seems to be tied to how self-serving he is. Melkor is clearly shown to be such a one.

So if Melkor played what he played not out of vanity, but to enrich the whole, it would not have been evil or turn to evil. That makes sense.
But the Ainur were the offspring of Eru's thought, so Melkor's vanity and lust for power had its source in Eru. This way, again, evil has its source in Eru.

I just keep sticking to this because Eru is always portrayed as being wholly good, and I don't think this is possible.