The defense offered up by Jack McCall at his trial in Deadwood was that the killing was a vengeance killing. According to the August 17th edition of The Inter Ocean McCall offered up this testimony

The prisoner was called upon to make a statement. He came down from the stage into the auditorium of the theater, and, with his right hand in the bosom of his shirt, his head thrown back, in a harsh, loud, and repulsive voice, with a bull-dog sort of bravado said: 'Well, men, I have but few words to say. Wild Bill killed my brother, and I killed him. Wild Bill threatened to kill me if I crossed his path. I am not sorry for what I have done. I would do the same thing over again.['] The prisoner then returned to his place on the stage.

Other testimony was brought up by the defense to bolster Jack's character while simultaneously demonizing Wild Bill as a fiend and "terror of every place in which he had resided."

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Was this enough to sway the jury or was there potential foul play, we don't, and probably never will, know for sure if there was. McCall says later, after being caught attempting to break jail, that Wild Bill's murder was procured by a man named Johnny Varnes. This is highly suspect and looks to me as a statement of a man who knew he messed up and was trying to get out of trouble.

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My own thoughts on why he was let go was because of who the principals were in this affair, and the not far removed events of the Civil War. McCall was a southerner from Kentucky with little to his name. Bill was a Yankee, a war hero to some, and well written about man who became a household name as an Indian fighter and lawman. I imagine there were enough Southerners on the jury to sway not guilty vote. This becomes even more believable to me when you consider that vengeance killings were a somewhat acceptable defense, doubly so in an illegal mining camp. All this, I believe, comes together for the jury to render a verdict of not guilty.