Police Activity has a video showing not just the shooting, but the lead up and aftermath.
It shows the desperate struggle of 6-8 officers rendering aid trying to save his life, and the near breakdown of the officer who shot him.
But its flagged to all hell and you're not even allowed to use the share function.
He was very unlucky. In my opinion his goal was to hide the gun and then comply with officers. He was just a dumb 13yr old who was in WAY over his head, and unfortunately died as a result. The cop had every reason to believe he was about to be fired on, and exercised the only option you have in that scenario.