In Nightcrawler, the injured gangster shoots down Rick when he comes in front of him, but when the same gangster comes out of the car, he sees Lou standing right in front of him, filming. Yet he does not do anything and limps away. Why didn't he shoot Lou?
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.
I don't think it has any proper explanation or motive. The answer could be opinion based. Suppose gangster didn't shoot Rick, then we could ask 'why didn't he shoot the damn camera-man who was filming his accident after having a gun?'
So I can come up with my own thinking. There is 2 strong reason to shoot Rick and to leave Louis. Rick was in the path from where gangster got out of his car, Louis wasn't. He was in a bit distance. Secondly, there was police siren when gangster was in eye-contact with Louis. He could think that he needs to escape first. Trapped, injured man can think in a million ways. Let us think like a gangster. Lying injured in a car just after the accident- "What the f? Who's filming me? Ahh! dhish dhish(shots fired) die idiot die. Now let's get out. Oh no! another one!! Why isn't he running!! Siren! Police! Aw, I need to run...." Maybe I would do the same thing :)
Here's my take on it. Right after watching the scene, three more reasons came to mind:
The guy's actions suggest he's a hardened criminal and quite probably a murderous psychopath. When he sees the weak, indecisive intern is in his way, he takes him out. But when he sees Lou, who is increasingly terrifying throughout this movie, he recognizes a fellow psychopathic mind and chooses to let him be.
Lou's defiant stance and stare also indicate that Lou is not afraid to die. So killing him would just be redundant, an empty gesture and a waste of bullets which the criminal desperately needs.
The guy is injured, the car is totalled and the police is seconds away. He suspects he's about to go down, so he goes out in a blaze of glory - and wants his last moments to be documented.