The allegations that a churchgoing, married father of two was Kansas' infamous BTK murderer have shaken the Hollywood image of the serial killer as disenfranchised loner. And it has people wondering just how many of their mild-mannered colleagues, spouses and fellow parishioners might secretly be monsters. Estimates of how many serial killers are operating in the United States at any given time are, like the killers themselves, all over the map. Jack Levin, who studies violence at Boston's Northeastern University, estimates conservatively that there are about 20 serial killers operating nationwide, accounting for about 200 victims a year. Ann Rule,...