Put yourself in this situation: You are responding to reports of multiple shots fired at 2:30am. You arrive on the scene, and one of the suspects takes off running. You chase after them. After chasing them several hundred feet, they stop. You have your weapon drawn- this individual is presumably armed with a handgun. You order them to stop and put their hands up. They quickly turn around towards you.
There is no time for a reasoned analysis of things: this is adrenaline and training and split-second muscle memory.
It's tragic. It's especially tragic that life events led to a 13-year-old being out at 2:30am with a 21-year-old parolee, shooting a handgun in an urban area. Blame the system, blame a lot of things, but this cop didn't do anything outside the boundaries of reasonable use of force. He couldn't know the kid had thrown the gun away. He has a reasonable expectation that the armed suspect that quickly turned around to face him could still be holding that weapon and had bad intentions.