He couldn't stand it. He'd been humiliated before everyone. As the oldest he had a place to keep before his fellows. And his humiliation had destroyed that! For wasn't his younger brother chosen to be king of this new world, king even over he? He trembled in rage at the thought of his younger brother ruling over him.
He'd only wanted to be like Atar. He had listened and watched the Music, and then invented his own to show that he too could create, to show all the younger ones that he deserved their admiration and reverence as Atar did. Now he had nothing: his rightful place had been taken from him, and given to Manwë.
Well, Melkor vowed, if he couldn't have what should have been his, he would do his best to see that Manwë didn't get it either. He would destroy it.
He would destroy it all.
And so the great fall...