So, he was both a Guatemalan national and a Metro police officer, eh?
Legal residents are eligible whether or not naturalized citizens.
Kind of like foreigners can sign up in the US military services.
I don’t know if it would even be legal (under current screwy regime) to ban them.
That said, why did they want the guns in Guatemala? A lot of folks in Mexico and Central America are getting guns because they want to defend themselves against the endemic drug cartels and other crime. The way of getting them was wrong, but not necessarily the intended use.