Few Americans are aware of a mysterious world that teaches civilians how to master the deadly sniper art — and it's only mouse clicks away. Through videos and Web sites, civilian snipers have attracted a cult following. "'One shot, one kill' is the slogan of the sniper," said Tom Diaz of the Violence Policy Center. A series of sniper shootings — eight of them fatal — in the Washington, D.C, region has area residents living in fear — which is exactly the way the sniper wants it. "There is a subculture, there is a civilian subculture in this country dedicated...