If he possessed the gun illegally, then I would hope he does get prosecuted - he broke the law. There are legal ways to possess a gun, and he should use one of those. If he didn't, then why shouldn't he be prosecuted? And if you don't like living in a location with strict laws on gun ownership, then you can always move.
I can't believe you think this would help. Yeah, there's a chance that having a gun would have prevented the carjacking. It's also far more likely that it would have turned into a shootout. I'd rather lose my car than risk my life. BTW, same thing with the Chicago story. Yeah, the burglar might have harmed them, but he probably wouldn't have - the vast majority of burglaries don't involve violence.