Your reasoning doesn't add up because the factor you are using (lax gun laws vs strict between states) is far too coarse to make any reasonable inference from.

California has tight gun restrictions. But high income cities, counties, and neighborhoods in California (i.e. Palo Alto, Marin, Pacific Heights) with intact community structures have very low gun violence rates by US standards.

Low income areas with a significant illicit drug trade, high rates of intra-community trauma, historical abuse by police, and a frayed community structure, have high rates of gun violence.

The availability of guns, purchased legally or illegally, has a more pronounced impact on the latter sort of area than the former because the latter type is more likely to produce interpersonal conflicts which escalate to the use of guns, because other means of dealing with the conflicts (police, the court system, community structures) are often less effective or available for them.

Not all that surprising. The low restriction states also tend to be low-urban-population proportion; many types of crime increasing in urban environments is a long observed thing across pretty much all human societies, for which density, relative anonymity, and Stark and omnipresent socioeconomic contrasts have all been suggested as contributing factors.

> In fact, those US counties with zero murders (about half) have the most lax gun control laws.

Well, sure, they also mostly have populations below (often by an order of magnitude or more) the level at which the expected number of murders at the national rate would be 1.