Ever since escaping from North Korea nearly two decades ago, Mr. Kim had led a comfortable life in Seoul. He enjoyed a successful career as a physical therapist. He was engaged to someone he loved.

But the new life that Kim had built for himself in South Korea started crumbling down some four years ago, according to the defector and his lawyer, when Pyongyang spies contacted him and threatened to harm his family still in the North if he didn’t do what they said.