If simply having a gun is good enough justification to shoot someone, then shooting every cop you see, because they have guns, would be justified. Or maybe it's not justification and incompetent trigger happy cops who are scared of shadows killed this guy unjustly.
If they should have shot anybody it's the first guy who says "he pulled a gun on us" and then proceeds to kick someone who's down. Or they could have directed their attention to the guys fighting and instigating and not the guy who was doing the opposite. The cop threw him to the ground, causing the gun to fall, and when he picks it up, still in the holster, and is walking away, him and his partner shoot him dead. Heroes.