In The Lost Road there are a few early glimpses of Melkor(then Alkar)'s story.
But Alkar, who had journeyed alone in the Void before the World, seeking to be free, desired the World to be a kingdom unto himself. Therefore he descended into it like a falling fire; and he made war upon the Lords, his brethren. But they established their mansions in the West, in Valinor, and shut him out; and they gave battle to him in the North, and they bound him, and the World had peace and grew exceeding fair.
While there aren't significant differences, we get Melkor's motivation and it's not to create or to destroy, it's not to spite Eru and the rest of the Valar, it's not even to make the World his kingdom, because that's a consequence of his motivation, not the motive itself: All Melkor wanted was to be free. Desire for freedom is why he sought the Secret Fire in the Void, it's why he wanted to rule Ea.
When Sauron pushes for the worship of Melkor in the Second Age, and he finally reveals the name of the one the Numenoreans are to worship, he says that 'his name is Melkor, Lord of All, Giver of Freedom'. And as Tolkien wrote, Sauron tried to have Melkor worshipped in Melkor's own terms. One may suppose that Melkor had styled himself as the Giver of Freedom in order to manipulate and to gain traction for his cause in the first place. But personally, I think that he genuinely saw himself that way. It's a valid interpretation to assume that Melkor might have initially wished to expand the freedom he desired for himself upon the peoples of Middle-earth. In the beginning he aimed for kingship rather than tyranny.
And there's a tragedy to it, isn't there? Reading about someone who wishes to think freely, to create freely, to act and to live freely and even to share this freedom with others... realising that there are limits to his freedom. Feeling that walls surround him and banging his head against them. Falling into a vicious cycle of corrupting and destroying in a futile attempt to experience agency through creation. Driving himself into a nihilistic madness that is the polar opposite of what he wished for himself at the start of it all...
Even in the published Silmarillion, I don’t interpret Melkor’s wanderings in the Void as evil in intent, though it’s obvious they were perceived as weird by his kin and estranged him from them. But that wasn’t yet the time of his fall, even in strict canonical terms, since it happened during the Music. I also think he simply yearned for more freedom, feeling uneasy with what Eru had assigned to him.
That’s why I’ve always wanted to know more about Eru’s intentions in creating Melkor and making him the strongest of the Ainur. I love the headcanon that Melkor was originally meant to be the king of Arda and it fits with him being the most powerful, sharing in all the other Valar’s domains. Maybe the problem was that Eru made him too strong, too ambitious, but also too hungry to rule and to be the sole source of authority… maybe he was, in some ways, too much like Eru himself. But unlike Eru, he lacked the ability to truly create, to truly own anything, he was doomed to remain only an instrument in Eru’s Music. And being the #1 instrument didn’t make it any better in his own eyes. His thoughts eventually led him to the conclusion that it’s "better to reign in Hell than serve in Heaven".
And despite Eru giving all his creations the ability to think and decide for themselves, he still caged them. There was no way for anyone to leave him and find a place where they could truly exist free, without constantly looking over their shoulder and wondering "But what if my creator wouldn’t approve of what I do???"