A famous case of this is https://en.wikipedia.org/wiki/Killing_of_Philando_Castile where the man identitified he had a concealed carry, the cop told him not reach for it, he started to say he wasn't, he was getting his license the officer asked for, with the officer cutting him off repeatedly and the officer shot him because he 'feared for his life'.
All they have to prove is that they fear for their life. It does not have to make sense, does not have to be 'justified', etc.