Iluvatar and Melkor.

[Hookbill the Goomba]

Ok, I got a copy of the Unabridged CD's of The Silmarillion. (DONT LAUGH! You all want one, if you haven’t got it!) Which was interesting, as I had just finished a re-reading of the book.
As I was listening to Ainulindalé a thought came into my mind; All of the Ainur preserved only that part of the mind of Iluvatar from which he / she (We have to be politically correct) was from. Now, If Evil came into the heart of Melkor, dose that suggest that Iluvatar was in part evil?
Moreover, This is strange seeing as Tolkien was a devoted Catholic, Iluvatar I assume to be an interpretation of God. Therefore It is a contradiction that this should occur. Is the fact that Melkor became Evil dew to That part of Eru's mind(I'll just say Eru its easier to type) or was it something else?
So, Melkor is evil, we know that. What of Sauron? Being corrupted by Melkor he "was only less evil in that for a time he served another". However, when Melkor was defeated with no hope of returning why would Sauron continue his evilness? Was it to for fill the will of his master? Did he wish the power for himself? Or was he corrupted beyond all hope of return? It could be all of these things, but we see , i think its in Akallabéth, that the seeds of Melkor's evil would stay in the world for ever.
It is my belief that Sauron perhaps was wrathful and wished revenge on the ones who had defeated his master, and corrupted by the power left to him by Morgoth, he went into deep evil. What do you think of both these matters?