Susan, the Good Samaritan law isnt a requirement-it simply protects those who choose to give aid from legal action, as I recall. Like, you cannot be held liable for someones death if you render aid and, as a result, that person dies, or suffers permanent damage, so long as you were acting in I guess would be called "good concience".
As for shooting someone; I have ALWAYS been taught that you continue to pull the trigger until the person or persons are no longer a threat. My perception of that would be when they stop moving. Granted, I would give verbal commands prior to pulling the trigger-once the trigger is pulled, there is no taking that back. So you either neutralize the threat, or you die trying. My hope is for the former. Sorry, but I couldnt bring myself to attempt to render aid to someone who was intent on causing me & mine harm...it may be cruel and callous, but I see no need to render aid to someone who intended to murder me, or cause me or mine harm.