In the Arbery case, the son admitted pointing his weapon at Arbury before Arbury made any move to threaten him. The moment he did that, Arbury had the right to defend himself by trying to take the weapon away from the son.
That's one reason why you should never point a gun at someone unless you attend to shoot them. The mere act of pointing that gun at someone is them the right to use legal deadly force against you.
Through the truck? Because Arbery was running at him around the truck. If I saw someone running at me, I would be pointing my gun at them too.
You and I have a different idea about "move to threaten him." I interpreted everything Arbrey did as threatening.