The trick was actually that Johnny joined the competition in the first place. I reckon folks'll get mighty suspicious and jealous when you're lugging around a golden fiddle.
We hear about Johnny's triumph, but I suspect that the story plays out in the devil's favor afterwards. He just wanted to make sure that this fantastic fiddler was at the center of the mayhem - taking a musician meant to spread merriment and turning him into a tool for chaos, maybe even rending him from this mortal coil in the process.
He shows up in Oh Brother Where Art Thou doesn't he?
I like "xitter" (pronounced 'shitter')