Bad Guy With Gun shoots and kills police officer. Good Guy With Gun shoots BGWG. Next police officer on scene sees GGWG holding rifle over body of slain officer and slain BGWG, and shoots and kills GGWG. So, yeah, GGWGs can help in rare circumstances, but having lots of unidentified people with guns can only lead to this sort of confusion. In the end the only real difference this GGWG made was getting himself accidentally shot and killed by police. This is why police wear uniforms and drive marked cars.

Colorado man who intervened after ambush on officer was fatally shot by police