So your reasoning is that if you follow somebody as part of your job and he turns around and beats you to death, you have no right to defend yourself in whatever way you can.
You ought to read the rap you are posting with an open mind. You should not try to represent that you understand either the intent or the letter of the law here.
If you just had your nose broken and the guy who broke your nose who is continuing to beat on you of you has you trapped and says he's going to kill you and you have a gun and you use it - that's self defense.
Your attacker in the heat of the moment can not have as much right to beat you to death as you have to defend yourself.
It makes no difference whether your job was to follow him or not to reduce neighborhood crime. It makes no difference whatsoever what caused him to start beating on you, or why he is upset with you.
It's called 'neighborhood watch' not 'neighborhood follow' for a reason.