On Evil and Good
“…Manwë wept and bowed his head. But at that last word of Fëanor: that at the least the Noldor should do deeds to live in song for ever, he raised his head, as one that hears a voice far off, and he said: ‘So shall it be! Dear-bought those songs shall be accounted, and yet shall be well-bought. For the price could be no other. Thus even as Eru spoke to us shall beauty not before conceived be brought into Eä, and evil yet good to have been.’
But Mandos said: ‘And yet remain evil.'”
–The Silmarillion, page 98.
“Then Ilúvatar spoke, and he said: ‘Mighty are the Ainur, and mightiest among them is Melkor; but that he may know, and all the Ainur, that I am Ilúvatar, those things that ye have sung, I will show them forth, that ye may see what ye have done. And thou, Melkor, shalt see that no theme may be played that hath not its uttermost source in me, nor can any alter the music in my despite. For he that attempteth this shall prove but mine instrument in the devising of things more wonderful, which he himself hath not imagined…And thou, Melkor, wilt discover all the secret thoughts of thy mind, and wilt perceive that they are but a part of the whole and tributary to its glory'”
–The Silmarillion, page 17.