youngunz4life, the accepted response is to shoot until the threat ceases. Shooting to kill, shooting to prevent potential future crime, and shooting to avoid prison over-crowding are not accepted reasons for an affirmative SD defense.
In the OP's case, the BG dropped at the first round, and ceased all threatening behavior. Shooting any more rounds at that point would, at best, have muddied the legal waters.
Now, if he'd trained to double-tap and re-assess, as many military and LEO types have been trained (though that paradigm has been shifting in recent years), and if he'd reflexively fired a double-tap but then ceased fire, he'd probably have been ok.
A lot depends on the DA; a lot depends on local politics and culture.
There have also been documented cases where very well-trained LEO's (SWAT firearms instructors, for instance) have emptied their weapons without realizing it, in the heat of the moment thinking they'd only fired five or six rounds.
I'm not saying to empty your weapon; I am saying that there are times when people may fire more than the minimum required without conscious decision to do so, because of fear or adrenaline. This can definitely complicate a legal defense.
Massad Ayoob has a lot of material on this kind of thing.
But my previous point was that laying a framework of "I advocate shooting until the BG can't overcrowd the prison or repeat the offense against somebody else" is unwise.