In most states you can’t use deadly force to arrest someone for the misdemeanor of trespassing.
However, possessing a firearm and being attacked can be a reasonable resort to use that firearm in lethal self-defense.
Not sure how GA law puts it, but use of force is justified provided you are not the aggressor during that specific instance if behavior.
From my viewing of the video, it sure appears that the junior McGrath was armed, and Arbery assaulted him. I didn’tnot see nor hear anything that led me to think that they were threatening him with lethal force.
Since it is legal to be in possession of a firearm openly, it is legal to attempt an arrest and it is legal to defend oneself from an attempt of disarmament (leading one to be in fear for their life if infact disarmed) it is therefore reasonable to use force, lethal force if need be, to stop the attack.
Simply to convict the guy of murder because he shot the other guy is unreasonable. As in all crime, elements must be met in order to get to that point.