I am not a lawyer and this is NOT legal advice. We should also remember that anyone can sue for anything, so I am not going to talk in regards to civil matters or Hawaii bias on how they look at things from a civil standpoint.
Some thoughts on the video....
If the weapon is legal and legally in the store, it does not matter who uses it in defense of their own life or the life of another. If the use of force fits the scenario, defense of life is #1 priority. If you are brought up on some other illegal weapons charge, that is separate matter. Dad giving son the weapon is gtg. We might also apply this to carrying concealed illegally which is a more extreme example. Say this happened on the street by someone who was carrying a weapon illegally. If the justification in use of deadly force was there, that criteria is met. Was the weapon carried legally? No. Can you be brought up on charges? Of course. Was your use of force appropriate (assuming it was), your still alive, you have legal precedence on use of force. May you be brought up on weapons charges? Sure. Are you alive? I am NOT advocating breaking any laws but just giving a more extreme hypothetical that might explain the store situation more clearly.
Think of other scenarios within the home, where family members gain access to guns that are not registered to them, nor are they old enough to own. They use it in self defense. GTG. I know a store differs from a home, but the same basis for self defense and use of force applies.
If you are legally able to shoot in defense of your life or the life of someone else, it does not matter where you shoot them, or their body orientation in regards to you? Facing you, back to you? If you reasonably believe that making yourself or your actions known prior, retreating or not taking action might place yourself or another in more danger of their life, research this one. Of course you should understand your backstop but that is a different concern.
Most people don't truly understand the actual dynamics of an armed encounter until well they are in an armed encounter and most people don't have adequate training which is obviously outlined by hit / miss ratio, even in regards to LE. .
Most people don't truly understand the dynamics in regards to ballistics, stopping power and the length of time that someone can actually maintain a fight based on their mental or chemical induced factors. Shy of an effective central nervous system interruption or if you get a psychological "tap out" by the assailant, there are no quick stops. Even if I explode someones ticker, it takes a minimum of 4 seconds to deny enough oxygen flow in and out of the brain to cause incapacitation. They have seen people last 15+ seconds. If your average, non-trained person can pull a trigger every quarter of a second, how many times can they potentially end your life also? Again due to lack knowledge and correct training, most people, even LE don't have the right mindset or utilize correct tactics in an armed encounter. I am not talking tactics like doing "Hondo rolls", but pure knowledge on employing and running the weapon on a potential lethal threat and at what point that threat no longer becomes a threat. Most don't understand that threats are still active much longer than they originally assume from their first encounter or exchange. This video is a good example of this. Not digging at the son as he did a good job, but it could have just as easily ended poorly with one lucky shot from the perp as he left the the door open for a lot of opportunity. The more the individual knows via training and education on the topic, the easier the ability for person to correctly articulate their actions and justification, but this is a deep topic in and of itself, so I will leave it there.
Again this is NOT legal advice and I am not a lawyer. Take this fwiw.