Another problem:
“However, a maniac does not care about the police, and if armed, may be resistant to even bullets, so if you shoot him, insure he is dead.”
As practical as this is, this is also ILLEGAL and will get you tossed in jail for murder. You shoot only until the threat stops. If the threat continues, you keep shooting. If there is no clear threat, do not shoot.
Modern forensics is more than good enough to tell if you shot someone while they were lying on the ground to finish them off.
Simple rules of engagement. However, if you should go for center of mass followed by headshots and if that should “happen” to kill them while they are upright, that’s another story.
I agree, don’t shoot a man in the back as he is running away or if they are down and not attacking.
Remember these words: “Officer, I was in fear for my life. Even after I shot him once, he kept coming so I kept shooting until he stopped.” And say NO MORE. Keep your story simple and do not stray or elaborate. There isn’t a jury in the land that would convict you.
All that is jurisdiction dependent, that is "threat to what" is dependent. In Texas it need not be a threat to your person or that of another.
Forensics can tell if you shot him while he was on the floor, they have a more difficult time telling if he was getting up.
Still, if he's down, you need to keep him "covered" and if he so much as twitches, shoot him again. Granny in Arlington Texas did that, she missed, but being already shot once, he wasn't interested in Granny or her granddaughter, he was headed for the door. Police had already been chasing him when he invaded the wrong house. They found him on bleeding all over the neighbor's deck. Granny had quite the mouth on her too, as documented by the 911 tape. Police action, besides chasing him into her house, was to take her gun. Replaced temporarily by one from a neighbor.