Re: I've seen other controversies like this, often involving real police and not a "watchman."
As you know, you have to perceive a threat to your life to use lethal force.
Running after someone who is fleeing, and then killing them, doesn't qualify.
I think his only defense would be "he went for my gun". It was either drawn, in a holster, or concealed. Either way the shooter put himself and his gun in range of the victim.
That's 'if' the kid went for the gun.