I'm pretty sure the policeman had street clothes on meaning his gun was probably under his clothing as a concealed weapon. The murderer told the citizen not a policeman that it was a family affair and he and his girl went back to the car as if the argument ended. The murderer pointed his gun on the citizen and didn't shoot it until he got in the car and shot through the window. The citizen had no time to answer an unsuspected shot through the window when it looked like the confrontation was over. I wonder if the whole deal was set up by the bad guy looking to do violence against anyone. He was confronted by a civilian he thought while he was roughing his female up, his right to do so, or it was a set up. I've got a feeling he due to his violence record and his public physically roughing his female around and an unexpected, dared confrontation by of all thinks a civilian, was perfect for him to enjoy another violence trip.