I can't resist. Nor can I resist the following round-about way of getting to my conclusion. Please be patient.
As Elrond said (and others have noted), nothing was evil in the beginning; not even Melkor.
Nerwen quotes the section in Ainulindalė that narrates Melkor's original descent into evil.
Quote:
|
desire grew hot within him
|
Melkor lost control of himself. He lost control of his desires. The desire began to control him. This is his first step in the descent into irrationality.
Sauron's descent into evil is not described as such. The closest we have to an accurate description of the nature of Sauron's evil is Gandalf's words during the "final debate": "His Eye is now straining towards us, blind almost to all else that is moving."
This shows that Sauron also had a desire (for the Ring) of which he was not in control. Nonetheless, Gandalf pretty much credits Sauron with weighing all chances to the finest point; which suggests rationality.
Melkor, it ought to be remembered, for all the personality with which Tolkien invests him, is a more elemental figure than Sauron. He represents fire (and ice) (to Manwė's air, Aulė's earth and Ulmo's water). Melkor's personality is necessarily more iconic.
So I guess I'm coming down on this little debate as saying you're trying to compare apples to oranges. Melkor cannot rightly be called a villain, even, as such a moniker is borrowed in from an altogether different genre than myth. Sauron can be called a villain, I suppose, since LotR is more or less a "romance". So apples and oranges.