John Joubert, Nebraska Boy Snatcher
Journalist Michael Finkel learned that fugitive and family killer Christian Longo was posing as him when he was arrested, so he approached Longo after his conviction and wrote True Story, a book about both the case and his experience of being impersonated by Longo.
The Adversary, by Emmanuel Carrère, who had corresponded with Jean-Claude Romand in France, is a more poignant story along these lines. Romand had begun his path into criminality with a small lie, when he said he'd passed his final medical exams. He got away with that, and through a succession of frauds, devised the persona of a humanitarian doctor and researcher for the World Health Organization. He offered "investment opportunities" to his family and friends, and with their money, he supported a nice lifestyle for his wife and two daughters. Then, pressured by questions he couldn't answer, he pretended to have cancer. When pressure mounted, on January 9, 1993, Romand murdered his wife, children, parents, and dog—allegedly to "spare" them the grief of realizing his deceptions. Psychiatrists decided that his lies had staved off despair. They diagnosed him as suffering from narcissism, mythomania, and an immature character.
Carrère assured Romand that he wanted to "understand as much as possible what had happened" and offered sympathy about Romand's situation, although he personally felt extreme repugnance over the crime. He claimed he wanted to "show the terrible forces at work." When Romand eventually accepted the invitation, Carrère found himself wondering what he'd gotten himself into. "Now the case and especially my interest in it rather disgusted me." He admitted to guilt over not feeling guilty, but went ahead with his plan, adopting the right tone of "pathetic and sympathetic gravity" to gain Romand's trust. It didn't hurt that he pretended to accept Romand as himself a victim. And he did end up feeling genuine sympathy for the man, but in light of Romand's life of lies and a certain amount of self-blindness, Carrère always wrestled with his ambivalence. In the end, Roman seemed to him a mixture of "blindness, cowardice, and distress." The writing of the story, in the end, "could only be a crime or a prayer."
On Joubert, Robert Ressler more or less had the last word.