The guards in the remote Libyan town of Zintan called their prisoner, Saif al-Islam Gaddafi, “our black box.” The second-oldest son of Muammar Gaddafi knew the secrets of Libya’s past, they said: Gaddafi’s deals with foreign governments and companies, the whereabouts of still-missing prisoners, the details of crimes committed during the regime’s failed attempt to crush this year’s popular revolt. Saif al-Islam, 39, had to be protected so Libyans and the world could learn what had really taken place. And for the guards in Zintan, a town of 50,000 atop a desert mountain, keeping Libya’s most-wanted man safe from the sort of frenzied opposition fighters who had apparently executed Saif’s father and brother Mutassim also offered a chance to patch up the country’s reputation for justice after the Oct. 20 killing of Muammar Gaddafi.