Ilúvatar and Melkor

[Kuruharan]

Another thing, where does it say Melkor is not obedient?

That would certainly be an interesting way of looking at it. At one point in his conversation with Tuor, Ulmo says that it is his part to seem to oppose the Valar in his help to the Children.
However, in my reading, Iluvatar's statements to Melkor after the end of the Music always seemed to have a tone of rebuke as if Melkor had erred.
Also, Melkor setting himself up as a supreme god in spite of Iluvatar does not strike me as being particularly obedient. I don't think that Iluvatar intended anyone to have pretentions of supplanting him.

Regardless of how "different" Melkor may have been, he caused untold suffering, not just for the Valar but for the Children as well. I think that considering their position they are fully justified in dismissing him as an enemy of the state.