I think this is all about paradox. Eru sets the music's theme. The Ainur play it, thus making choices within it - and in Melkor's case against it. But Eru's words clarify: "...And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me..." There you have what some call fate (Iluvatar as uttermost source) AND choice (Melkor chose to play themes counter to Iluvatar AND Manwe, for example, chose to play music within Eru's themes). So it's both.
As for Shagrat and the others, evil is in the mind of the beholder. I'll risk a little ire and say that we are far more orcish than in Tolkien's time and can't see our own badness for what it is when we see it in Shagrat.
|