Mr. Litton, however, could not legally buy a gun because he was a felon. Mr. Honea said investigators believed that the handgun that Mr. Litton used was a homemade, untraceable gun made of parts from different weapons, often called a “ghost gun.”
So, what did California's gun control regime accomplish here?
The California gun control kept the crazy murderer and convicted criminal from buying an arsenal. He had to buy parts and assemble them into a functioning firearm, something that has yet to be regulated because gun advocates complain that restricting access to parts is an unnecessary government overreach.
If the shooter (not murderer, hopefully) couldn't buy an arsenal, why didn't he build one? A single weapon appears to have been enough for him and it wasn't as if the attack stopped because his DIY gun was somehow significantly inferior to a store bought one.
We don't know all the details surrounding the guy or the incident, so I'm willing to table discussions about what he built and what regulations might have prevented it until we know more. But building a ghost gun is significantly more difficult than buying a legal firearm, and the ability to circumvent a regulation isn't an argument for removing the regulation. He wanted a gun to commit acts of violence, and he couldn't buy one. He found another way, and we should be looking at additional restrictions, not looking to criticize the ones that functioned as intended.
Also looking at one data point where the regulation didn't succeed in it's aim is survivorship bias. We're not considering any possible attempts at firearm misuse that were prevented because the regulation made it difficult or impossible to achieve.
That's fair, but neither can we be certain that these regulations prevent gun crimes or similar crimes with different weapons. Regardless, it's apparent that gun regulations as we know them lead to more incarceration and more incarceration does not decrease violent crime and may actually increase crime in some settings. (Interestingly, the shooter in this case was previously incarcerated, but not for any violent crimes.) Therefore, we have a reasonable basis for a hypothesis that gun control does more harm than good.
Apart from the fact that murder is different than possession of some contraband, what good is incarceration if it doesn't reduce crime or actually increases it?
We should be reducing incarceration however we can and decriminalizing firearms especially. Gun control regimes have significant carceral components to them. In New Jersey, for example, mere possession of a magazine with a capacity greater than ten rounds is a straight up felony even if you don't have a firearm to use it with. Incarceration, of course, puts antisocial pressure on society by way of family separation, mistrust of law enforcement, reduction of funds for pro social programs, etc. So people naturally respond by arming up. All we're doing is filling prisons.