"Deadly force is justifiable to prevent..." Your goal is not to kill your attacker, but to prevent his attack. The surest way to do that is to shoot for the largest, most vulnerable portion of his body which is visible to you, usually the chest. The fact that the portions of the body that are the most reliable fight-stoppers are also the most likely to result in the attacker's death is acceptable, but no more than that. If your attacker dies after completing the attack, you have failed. If he stops his attack you have succeeded, whether he lives or dies. The movie fantasy where you shoot him in the leg, or shoot his gun out of his hand, is just that: fantasy.