Joab

David's nephew and commander-in-chief for most of his reign. A brilliant tactician who won David's wars but had zero chill when it came to personal vendettas. He murdered Abner (2 Samuel 3:27), killed Absalom against David's orders (2 Samuel 18:14), and assassinated Amasa (2 Samuel 20:10). David's deathbed instructions to Solomon included dealing with Joab (1 Kings 2:5-6).