I posted my latest comment before this one prior to reading leeneia's post that included examples of badman named Willy or Bill. Now as to Bill, I've known a number of males name Bill. But-it just so happens, I've known none in the Biblical sense of that word

:o} or :o(

as the case may be.

As to your question "Turns out Railroad Bill's real name was Morris. How did they get Bill out of Morris?", a person's nickname doesn't have to have anything whatsoever to do with his or her "real" name. As has been noted in this thread, rhyming's very important when it comes to composing songs. And there's many more words that rhyme with "Bill" than rhyme with "Willy" {silly; dilly; hilly, chilly, filly-what else?} and as to Morris, forget it.