In Chapter 20. “A brag too soon,” Benhadad says,
“Hadad helping me, when I get done with you, there won’t be enough left of Samaria for each of my men to pick up a handful of dust.”
It’s his way of saying, “God helping me.” He is talking about Hadad, the supreme diety of the Syrians.
The ruler’s name is Ben-hadad because the Syrians called their ruler the “son” of Hadad. Son in Hebrew is “ben.” So it comes to us as “Benhadad.”