Regardless, from the facts presented in the article the shooting was justified.
Black gun- from a distance it looked like an actual firearm. Many BB guns and airsoft guns look extremely realistic. If you are expecting a firearm and see a BB gun, you might not give it a second glance. It’s not like the kid had a BB gun with Power Rangers on it or something!
Disobeyed command- the officer saw the boy pick up the gun and put it in his waistband. The officer told the boy to put his hands on his head. The boy did not do so and instead presented the gun. That it was not an actual firearm is irrelevant; the officer had reason to believe it was an actual fully-functional firearm and posed a danger to him and those around him. Nothing else really matters.
Could we find out some time in the future that there were other extenuating circumstances which should have made the officer believe that the boy was not holding a real gun? Yes. Could we find out that the boy wasn’t holding it properly or was holding it out to give it to the officer? Yes. Could those things change this from being a justified use of force to unjustified use of force? Yes. But right now it looks like the officer was justified.
Play stupid games, win stupid prizes.