I know Mississippi John Hurt sang "cruel Staggerlee" but I havent heard "mean ol'" although I've seen written versions like that.

Although Staggerlee was probably based on a real person he was also a legendary figure in African Americal folklore. He was sort of an evil Paul Bunyon who started San Francisco earthquake by pulling the water pipe in a saloon.

The way I heard the story is that Staggerlee sold his soul to the devil in exchange for a white Stetson hat. The hat had magic powers that that, among other things, protected Staggerlee from the law. When he lost the hat to Billy he lost his protection so they were able to arrest and execute him. I haven't heard or seen any version of the song that explicitly describes the hat as magic, though.

John Hurt's version has one verse about the trial:

Gentlemen of the Jury, what do think of that?
Staggerlee shot Billy DeLyon 'bout a five dollar Stetson hat
That bad man, Cruel Staggerlee