"violent behaviour is occurring worldwide, even in countries that don't have a NRA"

It occurs everywhere OK, and always has - but as has been pointed out in another thread, the figures for all types of murder of all sorts in the UK are lower than they have been for 30 years.

Gun killings are particularly likely to happen in certain societies, with certain social patterns and cultures, and the USA is very much to the fore among socially comparative countries when it comes to gun killings.
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Here is a link to a page giving a rundown of the gun regulations in Colorado. You can't really call them "gun controls", since they contain such brilliant provisions as these:

"Local governments are prohibited from maintaining a list or other record or database of persons who purchase or exchange firearm or leave them for repair or sale on consignment, and persons who transfer firearms unless they are federally licensed firearms dealers, or of the descriptions, including serial numbers, of such firearms.

"Local governments may not enact any ordinance, regulation, or other law that prohibits the sale, purchase, or possession of a firearm that a person may lawfully sell, purchase, or possess under state or federal law."