Roy, That isn't how the law works in this case.  Self-defense is legal. Being armed on your own property is legal. It is both legal and reasonable to arm yourself specifically to investigate a disturbance on your property. It is legal to be armed and verbally confront, without apparent verbal nor physical threat of physical force, deadly or otherwise, someone apparently trespassing. 

Also cutting against the idea that he was "looking for premeditated trouble," he allowed himself to be unjustifiably assaulted several times before ever producing the firearm and, per the account, only fired after that did not deter the assailant. Under the use of force statutes I can't find where he ever lost the required "mantle of innocence" for his use of force to be justifiable as self-defense. (assuming his perception of the threat and force used is found to be reasonable by the authorities or a grand jury given the totality of the circumstances.)