The defense offered up by Jack McCall at his trial in Deadwood was that the killing was a vengeance killing. According to the August 5, 1876 edition of the Black Hills Pioneer Jack's testimony in the trial is thus "Well, men, I have but a few words to say. Wild Bill killed my brother, and I killed him. Wild Bill threatened to kill me if I ever crossed his path again. I am not sorry for what I have done. I would do the same thing over again." Along with the Jack's testimony his lawyer, Joseph Miller, called a number of other witnesses to speak to McCall's character and to sully Hickok's reputation. The first witness called was a named named P. H. Smith, McCall's employer. Smith testifies "He[McCall] had been in his employ four months; that he was not a man of quarrelsome disposition; that he had always considered him a man of good character; that he (the witness) had been introduced to Wild Bill in Cheyenne, and drank with him; that the deceased had a bad reputation, and had been the terror of every place in which he had resided." Vengeance killings in the old west were sometimes looked on as justifiable. This is most probably the case for his being acquitted. Now while not as believable there is another theory that has been put forth, and I feel i should mention it. The other theory that gets floated around is that the jury was bribed to let McCall go. After McCall's arrest and indictment he offered up the story that he was a paid assassin tasked with killing Wild Bill. The Black Hills Pioneer from November 11, 1876 says that a warrant was issued for the arrest of John Varnes charged with "procuring the death of Wild Bill." The paper goes on to say that they believe the story to be untrue and a manufactured falsehood. However, if McCall was paid to kill him it would make some sense for the Varnes to see that he was released, perhaps as part of the contract for the killing. The fact that McCall doesn't make any mention of this later on leads me to believe that it is a falsehood. Hopefully this helps.