The last time Benjamin Barber saw Saif Qaddafi, in early December, they spent a cheerless evening together in London. Barber, a political scientist and board member of Saif’s Qaddafi International Charity and Development Foundation, was in town for a board meeting that was supposed to have taken place in Tripoli but, a week before, had been moved to England. Over an Italian dinner in Mayfair, he asked Saif why.

“I don’t feel comfortable in Tripoli,” the 38-year-old son of Colonel Muammar ­Qaddafi said. “I have too many enemies there right now.” Saif was in a desperate mood. For years he had pushed his way into his father’s chaotic political orbit, urging him to support reform in Libya. Muammar had obliged in the past—but recently he hadn’t. Allies of Saif’s had been arrested and businesses of his shut down. He had decamped and wasn’t sure he wanted to return. “He felt he was not welcome,” Barber says. “He’d been struggling for a long time.”

Barber urged him to push on. Libya was a vastly different country than it had been only a decade earlier, he assured him, thanks largely to Saif. It was a speech Saif had heard many times. He’d long worked with Barber and other academics, executives, consultants, and lobbyists to plot Libya’s future. They’d encouraged Saif, too, and had become partners in a campaign to revive his country and his family name, while he in turn worked with them to make Libya a supposed model of peaceful liberalization in the Arab world. He was what the region badly needed, his foreign boosters said. Saif sometimes agreed.

Two months after that dinner, with Libya in revolt, Muammar asked his favorite son to return home, which he did. Then, seemingly overnight, Saif became a new man: not the deliverer his supporters had hoped but someone indistinguishable from his father.

The second eldest of Muammar’s ­seven sons, Saif al-Islam Muammar al-Qaddafi was born in 1972, three years after his father took...