TBF the fact he'd already hit him multiple times is what probably makes the ninth shot irrelevant, legally.
The first lot of shots are clearly legal self-defence (of himself and others) and would be in any jurisdiction I am aware of - a bloke pointing a gun at multiple people in a store, robbing them and in the US context of mass shootings being a daily event could absolutely be shot in the back when the opportunity presents itself.
The ninth one (the "execution" shot) would be really difficult if not impossible to get a murder charge out of because the defence would almost certainly argue that the eight which preceeded it killed him. Then there is all the emotional disturbance, the "I thought he was going to get up and had another gun", "I am just a civilian and I was terrified for my and my friends life" and so on. I am also not sure it was that intentional either TBF, he lets it go just as he is standing back up with the bad guy's gun as if he still had his finger on the trigger.