[Daniel Telcontar]

You have all spoken well, and I apologize if my arguments have already been posted. But I believe that Melkor was evil by will, because:
He was the greatest of the ainur, so it must have made him mad when Manwë became the highest ranking, because he understood Eru better.
if it was because he was enviuos, I doubt that it was something in his nature. Eru had chosen Manwë to be his closest servant;
If Melkor was like Eru in mind, he would have been pleased, hence I do not believe that he was evil by nature, but something he chose because he did not follow Eru faithfully, but walked his own path.