Agreed, and a shooting review board may ding him on that count, and a wrongful death case against the city may be filed. But put yourself in the cop's place: you respond to an active shooter situation without knowing what else is happening at the time. You arrive, you see an armed person holding a rifle. You get out of your patrol car and . . . what? Tell him to raise his hands? If he is the shooter (a logical assumption), then you will be shot at. So, no, you shoot. If you don't, you could be the next victim. Things happen really quickly when adrenaline kicks in."Person holding a gun" does not seem like sufficient cause for shooting the guy."Person holding a gun" does not seem like sufficient cause for shooting the guy.
My take on it is "why was the good guy holding the rifle?" I was told many years ago: if you are involved in a shooting, put the gun down and sit down with your hands visible so the cops don't shoot you when they arrive. You will be roughed up and cuffed and thrown in the back of a car. You will be taken to jail and booked. Your best bet is to be as harmless looking as possible and don't try to argue with the police when they get there.