The words “O mine enemy,” may refer partly to the old antagonism (marginal reference; 1 Kings 17:1; 1 Kings 19:2-3); but the feeling which it expresses is rather that of present oppositions - the opposition between good and evil, light and darkness John 3:20.
Thou hast sold thyself to work evil - Compare the marginal references. The metaphor is taken from the practice of men’s selling themselves into slavery, and so giving themselves wholly up to work the will of their master. This was a widespread custom in the ancient world.