Albert Barnes Commentary 1 Kings 21:20

"And Ahab said to Elijah, Hast thou found me, O mine enemy? And he answered, I have found thee, because thou hast sold thyself to do that which is evil in the sight of Jehovah." — 1 Kings 21:20 (ASV)

The words “O my enemy” may refer partly to the old antagonism (1 Kings 17:1; 1 Kings 19:2–3), but the feeling it expresses is more that of present opposition—the opposition between good and evil, light and darkness (John 3:20).

You have sold yourself to do evil—The metaphor is taken from the practice of people selling themselves into slavery, thus giving themselves completely over to do the will of their master. This was a widespread custom in the ancient world.