First off, who is to say having a firearm in the home renders someone other than "normal people"? In my State, not having a firearm in your home is abnormal, if you consider the majority "normal". Here, building walls around your property is only an accumulator for snow drifts in the winter and a grand way to cut off any summer breezes. Better to be able to see what is going on and act appropriately than cower behind a wall and be surprised. Note also, we have some of the lowest rates of violent and property crime in the nation..
That said, If the homeowner stepped outside and shot the fleeing kid in the back, it is murder. There is no justification for shooting someone who has committed no violent act and is retreating.
Alternately, other factors (death threats from another source, for instance) nay have prompted the homeowner to respond inappropriately to a mere prank. If this can be shown in court, the homeowner may have a defense.
The most dangerous people in the world are simply scared. To try and point the finger one way or the other in the absence of the full picture is being a bit premature.
I regularly have teens show up on my doorstep, but then, I have a teenage granddaughter. Some of these guys (and girls) look pretty freaky by our generation's standards, but their eyes are clear, and under the trappings of a youthful search for identity are some bright, decent adults-to-be.
I don't come to the door with gun in hand (I have other items in reach which will give me time to upgrade if necessary). But if someone tries to break that door down, the picture changes significantly.
Mistakes were made on both sides here. The kid should not have been ringing this guy's doorbell, for one.. The guy should not have shot the kid, for another... The courts will have to sort it out from there. Damned shame for all involved.
What I suggested was that certain "abnormal" people might well lie in wait for the unwary innocent person to foolishly ring the doorbell.