That is why you say nothing to the police. You let your lawyer do the talking, because it is his job to keep you out of trouble.
Well in this case the problem was that he followed the perp out to the parking lot. If he’d shot the perp in the act that would have been more defensible. Fortunately the cops were sympathetic to his situation and accepted his claim that he was afraid for his wife’s safety even though she was not present because she was on her way over. Who knows whether there was any truth to it. The cops are just about the only authorities who you can count on to act reasonably these days. They threw him a lifeline.
If you ever have to actually use your firearm; immediately shut up, and lawyer-up. You have the right to remain silent...