[Captain Walker]

He shot him 9 times because that was all the bullets the gun would hold. When someone is trying to kill you you must do all you can to eliminate the threat, as long as the perp had a gun in his hand he was a threat, the shooter had no way of knowing the gun was only a BB gun.

There's a related thread where all nine shots are shown; discussing the nine shots without seeing the nine shots is pointless. (You would have known that the shooter had already picked up the perp's gun before he fired the last round.)

[Glenmore]

All but the last shot could be called self defense; his best legal defense for the last one might be to claim the guy was already dead.