This question has been asked before, but it seems the most reasonable answer is this, from @chaitanya89:
The way I've seen it is, As soon as the bad guy's car crashed, Lou took his camera and started filming and Lou observed that the bad guy is injured and gaining senses and recovering, he wanted to use this situation and kill his partner. So, he backed off and asked his partner to come shoot with steady hands and zoom. Unlucky for the partner, when he reached in front of the car, the bad guy recovered and not thinking straight and shot Rick, at this point, gangster would have shot Lou if he was there. But after shooting Rick, when he saw Lou standing there filming without a budge, the bad guy was surprised for Lou's audacity and few moments later Police arrived and the gangster's main concern is to deal with the police who shot the gangster's partner down.
You can read more on this page.
Personally, I would see it as the gangster was coming to his senses after the crash and Lou realized that and backed off towards the cop car, so he could hide behind it if he saw the gangster pull his gun. He couldn't necessarily know what the gangster would do once he comes to, but took a chance and told Rick to tape the incident, knowing that he very well could be shot.