Do not attempt to kick the gun out his hand for 2 reasons.
1.) It's dangerous to approach a downed BG. What if he's faking? Or wakes up right as you get to him? Even if you manage to shoot him before he can do any harm to you, now the evidence shows that you shot him, walked right up to him, and delivered a coup de grace. You will go to jail for a very long time.
2.) The gun still in his hand is excellent evidence that you were in fear for your life. This helps you avoid going to jail for a very long time.
After he goes down, move to cover, call the police, and while you are waiting for them to arrive do one of the two following things:
A) If you are somewhere secure where the police can only approach from one or two directions, and you know that you can see them coming before they see you, keep your gun trained on the downed BG. When you see the police coming, put the gun down or reholster.
B) If you are in public, re-holster. You can leave your hand on the gun so that you can quickly re-draw and fire if he becomes a threat again. When the police arrive, slowly raise you hands, leaving the gun in your holster.
Obviously, in wither case, if he becomes a threat again, shoot him again until the threat stops. But whatever you do, you DON'T want to be standing there with a gun pointed at him when the police arrive.
I would shoot till I think that I had killed the attacker
So what do you do if you shoot him, he falls over and stops attacking you, but is obviously still alive?
Shoot to stop the threat. Nothing more, nothing less.