Unless the person had was attempting to use the gun to harm this man, simply possession of a firearm by someone, is not justification for shooting in self defense.
Given they haven’t shown any video of this situation, simple seeing someone has a gun on their person, even if they steal a bottle of water does not give you the right to use deadly force against them legally.
If this kid went for the gun and the store owner shot him, no problem. If however this guy shot without provocation, and no, stealing of a bottle of water is not provocation for shooting the thief, even if they have a gun on their person.
I am more than happy to wait to see if there is more information coming about this, if the thief went for his gun, the store owner acted in self defense. If he didn’t he committed murder.
All he had to do is yell, “It’s comin’ right for us!”