The nation demonstrated again last week how resolute it can be when threatened by murderous terrorists — and how helpless when ordered to heel by smug lobbyists for the gun industry. Tamerlan and Dzhokhar Tsarnaev’s deadly rampage through the Boston area provoked not fear but defiance. Even before one brother was killed and the other captured, the city was impatient to get back to normal, eager to show the world that unspeakable violence might shock, sadden and enrage but never intimidate.