Well at least I am with you drigel!
I also think that it was not against any rule for Melkor to introduce his own idea into the music. That it did not fit the first theme of Eru was unconvenient and Melkor should hve marked it himself and stop introducing it. But such perfection was not asked from him since Eru still smiled introducing the second theme.
But here I begin to disagree with drigel: the sconde theme did define what was whished for by Eru. To continue with a theme alien to Erus first or seconde theme was acting deliberatly against the will of Eru, wich I would asume to be evil. Wich is then refelced by Erus stern face.
The third theme was than no longer to define what Erus police was, it was his way to deal with the effect of Melkors continued evil behavior. The theme was succesful because it found its way to get around the theme of Melkor and even benefit from that theme. But Melkor did not learn his lesson. Therefore Eru did stop the music in the end to teach him and the other in plain words.(
)
That's how I read the Ainulindal. Thus I would say evil arose with Melkor continuing his theme after Erus first intervetion.
Respectfully
Findegil