If the description is accurate, this was probably a bad decision on the Father's part: cops see someone aggressively move towards a gun (and any attempt to disarm would, by definition, have to be aggressive) and they will perceive it as a threat.
Pretty sad... the Father is dead and the kid's life is ruined, even if there are no charges or a mandatory stay at a psychiatric ward.
Look, it's simple: cops are trained to respond in a Pavlovian fashion: pull the trigger repeatedly until the magazine is empty; repeat until threat it eliminated. Now, maybe we should change the way cops are trained, but that is another topic.