BOSTON — It was a blow the immigrant boxer could not withstand: after capturing his second consecutive title as the Golden Gloves heavyweight champion of New England in 2010, Tamerlan Anzorovich Tsarnaev, 23, was barred from the national Tournament of Champions because he was not a United States citizen. The cocksure fighter, a flamboyant dresser partial to white fur and snakeskin, had been looking forward to redeeming the loss he suffered the previous year in the first round, when the judges awarded his opponent the decision, drawing boos from spectators who considered Mr. Tsarnaev dominant. From one year to the...