I think that John Walton was being a jerk here. You don't mess around with the police chief that way.
I know that his pride was really important, but he put everybody else through the wringer for it.
But then, he's shown so many admirable traits throughout, he's entitled to be a jerk now and then to keep his character human.
I don't think John was really trying to be a jerk, and I don't think he was "messing" with the Sherrif. I don't think John was obligated to make a statement until proven guilty. Johns defensive behavior may have seemed over the top, but John Walton was an extremely proud an honest man, and in his mind, being accused and being suspected of theft was the worst thing a person could do to him. It was a huge insult to his pride and integrity. I know from experience that it is a very unsettling and stressful thing to be wrongfully accused, and it does bring up the defences.