Stalking from home to home, a U.S. Army sergeant methodically killed at least 16 civilians, nine of them children, in a rural stretch of southern Afghanistan early Sunday, igniting fears of a new wave of anti-American hostility, Afghan and U.S. officials said. Residents of three villages in the Panjwai District of Kandahar province described a terrifying string of attacks in which the soldier, who had walked more than a mile from his base, tried door after door, eventually breaking in to kill within three separate houses.