I agree with everything you said, however

> The more likely cause is the legal structure around gun ownership in the United States.

Gun ownership may be an enabler in mass shootings, but I don't believe it's the cause. Merely having a gun doesn't automatically make you a mass murderer. There are other underlying issues there. Not that that means the U.S. shouldn't do something about the legal structure around gun ownership. It's probably going to be much much harder to actually address the underlying cause.