It cuts both ways. If you kill someone in self defense, you could still get prosecuted, and it could have been a good shoot. Things get murky with civilian shoots, and prosecutors make bad calls. An instant death penalty might work against you.
Once a cop undergoes a background check, gets trained, and someone kills him while on duty, it is much less murky than a civilian shoot.
I happen to be a rather well qualified veteran. You think some cop has more skills and training based on some POST course and is more “reliable” based on an ENTNAC or something? Funny shit there. You don’t know what you don’t know friend.
My wife has no credentials and has as great a right to (protection under than law) life as any other person, including a head of state. That is the American expectation.