Maybe another non-biblical allusion, but could Randall Flagg be somewhat inspired by Charles Manson? It’s a loose idea, but they both had darkly charismatic personalities that allowed them to gain followers to help carry out some dark motive. Manson also kind of saw himself as some kind of prophet of the apocalypse, which is essentially what RF is. Also adding to it are the buttons on RF’s denim jacket, especially the one with the pig on it, “Eat your Pork” I think the quote on it was, I say this because Manson had some ideology that The Beatles were the horseman of the apocalypse or something, and had his own crazy interpretations of some of the white album songs, namely Piggies, which I’m pretty sure was one of the ones that was quoted in the blood of the innocent victims he told his followers to murder. Helter Skelter was also published in the 70’s, and King probably read it.