Doctor Franny Krieg was a former colleague of Doctor Joan Watson, and both a criminal and victim.
Biography
At some point in her career, Doctor Krieg decided to make extra money by stealing details about the medical licenses of various doctors she worked with, as well as altering their contact details on certain web sites. As a result, she was able to write out multiple prescriptions for otherwise dangerous drugs for various addicts who would otherwise have been rejected from other doctors. One of the identities used was Doctor Joan Watson, who had officially retired from medicine years ago but still kept her license valid.
Maintaining her regular practice, Doctor Krieg eventually became concerned about her patient Ethan Moore, who had been allegedly diagnosed as suffering from some unspecified immune system deficiency as a small child. When questioning Ethan's mother, Marla, Doctor Krieg was unsatisfied with her explanation for her son's illness. Further clues were provided when one of Doctor Krieg's fake identities received a voicemail message from Marla claiming that she was looking for a new doctor for Ethan, only for the details provided to Doctor Krieg's other identity about Ethan's illness being completely different from those that had been provided to Doctor Krieg herself. With this information, Doctor Krieg realized that Ethan was actually a victim of Munchausen syndrome by proxy; Marla was stealing her father's heart medication and injecting Ethan with it to make him believe that he was ill when in reality there was nothing wrong with him.
Despite knowing that she would most likely lose her medical license once the truth about her actions came out, Doctor Krieg arranged a meeting with Ethan where she told him of her suspicions about what his mother was doing to him. However, unable to cope with the revelation, Ethan was able to make contact with one of Doctor Krieg's addict patients and arrange for his help in killing his mother and Doctor Krieg during a later appointment.
The death was uncovered when Sherlock Holmes and Joan Watson found out about Krieg's illegal prescriptions, tracking Krieg down based on the paperwork she was using, which included reference to Joan's Chinese name as a middle name when she had only used that for a few key hospitals. The investigators initially assumed that Doctor Krieg was the target, but their research soon exposed what Marla had done. In a confrontation with Ethan after his arrest, Joan noted that, even if she could understand his anger at his mother, Doctor Krieg had risked arrest and the loss of her medical license by exposing her actions simply to try and help Ethan, only for him to kill her because he couldn't face what she had told him.