Melkor; Evil by Will or Evil by Nature?

[piosenniel]

Tolkien's faith, therefore, allows for a narrative of Creation and Fall in which the archetypal manifestation of Evil is consistent. And the telling of a story (in the tradition of the great myths he so admired) demands that a pantheon of archetypal (and in differing degrees oppositional) forces (and characters) is present.

Is the being Melkor the archetypal manifestation of evil, an objective reality, so to speak. Or is the archetypal manifestation of evil Melkor's actions which are a negation of the Creation of Eru. If he is the archetypal manifestation of evil, is he fated to always be evil. If it's Melkor's actions which are evil because they are a negation of the 'good', then is there the possibility for him to choose a different course of action given that he has an abundance of time in which to reflect and re-choose.