Gandalf says: "For nothing is
evil in the beginning." Also
Tolkien writes that the
Maia Sauron was not evil in the beginning but corrupted by
Melkor.
As to whether Melkor was 'originally' evil, no he was not. The
Ainulindale itself makes this clear and pinpoints the spot where he turned to evil.
"But now
Ilúvatar sat and hearkened, and
for a great while it seemed good to him, for in the music there were no flaws. But as the theme progressed,
it came into the heart of Melkor to interweave matters of his own imagining that were not in accord with the theme of Ilúvatar;
The moment Melkor chose to go his own way was the moment he departed from good and turned to evil.
As to HoME Vol. 1 "Unfinished
tales", it provides the slightly revised version of the beginning but is essentially the same.
"But now Ilúvatar sat and hearkened, and
for a great while it seemed very good to him, for the flaws in that music were few, and it seemed to him the
Ainur had learnt much and well. But as the great theme progressed
it came into the heart of Melko to interweave matters of his own vain imagining that were not fitting to that great theme of Ilúvatar."
So again, as we read also, at first "they all learned together" then
Eru brought them "the great themes".
Before anything of 'real consequence' had happened (some might say) Melko chose, and fell into evil.
Was he evil from the beginning? No. Did his pride and arrogance take long to destroy him? Nope. (whatever 'long' may be in the terms of the beginning and the time up until the unified Singing.) As long as they were learning 'individually' Melko was fine. As soon as he needed to 'work together with others' he was a hopeless disaster.
