I'm not saying it is at all, but they didn't know he he'd thrown his gun away. You can barely just make it out on the video when it's slowed down/paused.
The most important issue is they didn't give him time to react, but it's easy to say in hindsight. I think in those situations the kid should have just stood there with his back to them, drop the gun on the floor so they can see it. Wait for them to arrest you.
Anyway it's a very sad situation but let's not pretend he was a 13 year old angel. If you're old enough to roam the streets with a gun unfortunately you'll get caught up with trouble.
Would you like to reread that sentence and try again?