If Halbrand is a Man and a Bad Guy, then he is most likely one of the Nine Men who will be given Rings of Power by Sauron. They will become entirely corrupted and end up as formless shadows, the Nazgûl, or Ringwraiths. We know very little about who these Men were before they were given their Rings, but we do know that they were warriors and kings. If The Rings of Power is about the forging and wielding of Tolkien’s twenty great Rings (three for Elves, seven for Dwarves, nine for Men, and one for Sauron), then it stands to reason that we are likely to meet the Men who will eventually become the Nazgûl before they get their Rings, while they are still human.
We only know the human name of one of the Ringwraiths, an Easterling king called Khamûl. He is the Ringwraith who first rides into the Shire, and who famously sniffs for the hobbits while they cower under a great tree root. It’s possible that we will meet him in human form at some point during the series. Of the other nine, all we know is that three of them were Númenoreans. But we do know something of the later activities of their captain, the Ringwraith who stabbed Frodo on Weathertop and was eventually killed by Éowyn at the Battle of the Pelennor Fields.
The character is called “the Witch-King of Angmar” because he founded the evil kingdom of Angmar, full of dark magic, in the north of Middle-earth, which he ruled for many years until he was defeated by Gondorian Captain Eärnur and Elf Glorfindel, and retreated to Mordor. The character doesn’t necessarily have to have been a king as a human, but it would make a lot of sense if he was – he would already be a leader and a warrior.
The Ringwraiths also gain unnatural long life (of a sort) through their Rings, and Halbrand’s foremost quality so far is that he is a survivor. His determination to survive even at others’ expense, his violent tendencies, and his position of power in the land that is going to become Mordor all suggest that he is a prime candidate to be given one of the Rings of Power and entirely corrupted by it, once he and Galadriel manage to make their way out of Númenor to catch up with her friends Elrond and Celebrimbor.
Whether he’s Sauron, the Witch-King of Angmar, or just some guy, Halbrand is a character to keep an eye on going forward.