raywest

Answer: Florence Nightingale is considered the founder of modern nursing and nurses are often called "nightingales." It has nothing to do with Jack's mother telling on him or "singing like a bird." Whether or not Jack killed his mother is left ambiguous, but it does seem like that's what they are suggesting. In the film we are told Mary Finelli was his first victim. However, they seem to imply that Jack's mother's killing was not associated with the "Nightingale Murders." Had the police connected her murder to the "Nightingale Murders", it night had led them to Jack. So Jack changed his M.O. after killing his mom, which I take as targeting younger nurses. In the film, Jack's dad tells John that Jack got caught up in the Knapp Commission, which was an investigation into police corruption. Jack went to prison, which is why he stopped killing. In the film, it's suggested that perhaps the killer (before they knew it was Jack) was taken off the streets (i.e. sent to prison for a crime not related to the killings) and smartened up once he got out of prison. There's no suggestion that age or lack of testosterone is what stopped Jack from killing after getting out of prison. Of course, it's also possible he did kill again, but due to the gap in killings, those murders aren't related to the "Nightingale Murders." But all that is speculation since they don't really reveal any of that information and it's not central to the plot.

Bishop73

Answer: I can answer part of your question. Most serial killers are under the age of 50. Testosterone levels drop after that age, and it is thought that this may reduce a psychopath's urge to kill. The nightingale reference may be the same as the slang term, "to sing like a bird," which means someone gives up information easily. Jack's mother may have provided info about her son, either knowingly or unknowingly.

raywest ★

Answer: Or maybe Jack's mother abused him as a child, and that's why he hated her and killed her, and then became a psycho serial killer. He continued to kill other nurses because his mother was a nurse.