RangerSteve mentions the possible "historical background" to Cat Ballou, but as far as the movie itself was concerned it was actually based on a novel of the same name by Roy Chanslor (who also wrote a novel called "Johnny Guitar" which was the basis for the movie of the same name starring Joan Crawford, Mercedes McCambridge, and Sterling Hayden).

Believe it or not, Chanslor's novel is actually serious! It's a real soapy melodrama. I've never counted all the verses of the "Ballad" in the novel itself, but it's got to be a couple dozen or more. Here's the finale:

>>There's a empty noose a-danglin',
>>High upon the gallows tree,
>>But Cat Ballou's a-ridin',
>>With her true love wild and free!

Roy Chanslor. The best proof that the book isn't always as good as the movie.