Only if the kid was shot in the back.
Part of the responsibility of owning a gun is the training when to use it.
Shooting someone in the back while fleeing is illegal
That's not the "only" situation where it would be at last manslaughter, as I'm sure you already know. If I just throw open the door and shoot the next person that rings my doorbell - as tempting as it is when the JW's come around ;) - the fact that I shot them in the face/chest is not going to help me when it comes time to discuss what I did in court. The response has to be proportional to the threat, and if it turns out that this guy was under no real threat at all, he's in big trouble - and that business of "I felt my life was in danger" won't save him if the circumstances were such that it was basically impossible for his life to be in danger from this kid. You have to have at least some rational reason to feel that way, even if it's a flimsy reason.
Shooting someone in the back while fleeing is illegal
I agree. But you will find that some posters on FR believe that training is "infringement" and think the poor kid got what he deserved. Sad.