When A Patient Kills Her Therapist

Susan and Felix Polk did raise a lot of eyebrows when they announced their engagement, and it was not solely because of the age difference. The two of them started their love affair when she was just a troubled teenager and he was married to another woman. Unfortunately, their story didn't have a happy ending. Susan was a mental patient after all, and it seemed like Felix simply couldn't help her, no matter what he tried.
So what drove this woman to murder her husband? How did their relationship really start and was it ever healthy? Or did Dr. Felix Polk use his authority to control Susan from a very early age, resulting in her breakdown?