It's not that simple. Using a gun as a threat to get your vehicle back, still leaves the option of not using it, which seems the intended goal since the owner didn't shoot the thief right away which is always the move to make if you intend to shoot an armed person, rather than waiting to see if they will draw and fire.
Possibly the couple were going in the mall, or possibly leaving, or possibly snorting lines of cocaine. All we know for certain is this owner saw an opportunity to try to get his vehicle back, to keep the thief there until police arrived.
The thief should have complied with that, accepted the consequences for his actions. Instead he shot the owner to try to get out of a felony charge, created the kill or be killed scenario after the owner had established that if he had wanted to shoot the thief, he already could have.
It's not that I like the outcome, do agree with others that I'd pursue other means of getting the vehicle back like calling police and telling them I'm following them, IF I had a vehicle to do so which we still don't know. Maybe he had already called police and they told him, "sorry too busy, go pound sand".
Who is really the one who values a pile of plastic and metal over a life? The thief, because he valued it enough to steal it, then USED a deadly weapon trying to get away with that. Shooting at the owner of a vehicle you stole, when that owner is armed? Thief had a death wish, dead or alive you're not taking me in.