It's limited to citizens because the Second Amendment refers to "the right of the People to keep and bear", meaning the body of citizenry. No alien has the "right" to own a firearm in the U.S.
The NBC TV show West Wing, in reruns on the local ABC affiliate (shows from a couple of years ago) slimed Texas's concealed-carry laws with an episode in which someone stood up in a Texas church and opened fire, whereupon someone else with a CCW license also stood up and opened fire at the bad guy -- but missed and killed a 9-year-old girl (Democratic pity party: someone has to die!).
It hasn't happened, but Worst Wing's writers had a high old time putting President Bartlett up on his high horse, slapping down his Texan VP on the subject. It's the gun-grabbers' dream scenario: "more harm to let them carry". Mind you, the scenario was identical to the Charles Hennard and Larry Ashcroft shootings in Texas; Suzanna Gratia Hupp was testifying about the Hennard incident in Killeen. Larry Ashcroft, whose name usually doesn't escape my keyboard or my lips, was a Phineas Priest, a kind of right-wing assassin, who went after the Baptists because the Southern Baptist Convention had "compromised" its segregationist theology of 50 years ago. He shot up a Sunday-school meeting, then killed himself with a curse on his lips.