Paul John Knowles: The Casanova Killer
That fact that Sandy Fawkes was a journalist probably saved her life, because the idea of being immortalized in a book probably appealed to Knowles's narcissistic side. When Fawkes first met him, she immediately informed him about her work as a ploy to keep him at arm's length, but she inadvertently gave him an idea, perhaps making herself functional for him in a way that was different from his other victims.
Killing Time is a quick read, with no real insight or analysis, but it definitely documents the final days of Paul John Knowles, including the indignities of his inadequate sexual performance. Yet with the knowledge that impotence can trigger homicidal anger, it's likely that her ability to tolerate his inept performance further endeared her to him. He told her that he was having the best time of his life, and that may well have been true. He was finally with a woman who seemed to accept him for what he was, and who was there to basically provide him with company and the illusion that he had a "normal" life.
She seems to think that in the end, once he knew that she knew who he was, he might come back to kill her, but that may simply have been dramatic license on her part. There's no reason to think, if he spared her life because he wanted a book written about him, that he would have eventually murdered her. In fact, once he was arrested, they briefly encountered each other, and he smiled at her and told her she "looked good." He may have felt some genuine affection for her, and he was also probably certain that she would follow through and write the book -- which for her would be the story of a lifetime.