As darkness enveloped Stockholm one evening last May, the city set into its nocturnal rhythm and its criminal underworld came alive. In a home in Västerås, a lakeside city 60 miles west of the capital, a mother completed her nighttime routine, comforted by the sight of her 15-year-old son safely in bed. But, just before midnight, the teenager silently rose and slipped out. A job awaited him, one he believed would be over before daybreak, well ahead of his national math examination.