[Eaker]

Forensics are well nigh supernatural today and if you keep on shooting a downed assailant you will get into a heap of trouble because it will be obvious to medical investigators that you did.

One shoots until the threat stops being a threat. If they are down and you still consider them a threat keep shooting.

The law say a doctor not you have to call "time of death" for a reason.

[HiTech RedNeck]

And you are going to have to spend far more in defense costs and cool your heels longer in jail for trial if the forensics show you shot a fallen assailant in the back.