Why is Melkor so utterly evil??

[Mhoram]

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, for he sought therein to increase the power and glory of the part assigned to himself.

I don't think that he was evil to start with, but he was proud. He sought to create things of his own and order them according to his on ideas, he was wrong to do so but I feel some sympathy for him here. I'm sure that Yoda would have something to say about pride. Something like "Pride leads to Obsession, Obsession leads to Anger, Anger leads to Hate, Hate leads to Suffering, emmmmmm." Not evil to begin with but his character set him on a road that would lead to it.