I think I can make something good out of this one...
A woman's entire family has been killed, and she was wrongfully accused of the murders. The evidence against her was too strong, so her lawyer went against her wishes and used an insanity defense. For the past year, she has been in a mental institution. One day, she decides that she will try to escape and clear her name by finding who really did kill her family. During her year in the institution, she met another "patient" who seemed normal, and they became good friends out of necessity. Together, they set up a plan to trick the guards and rejoin the outside world.
The two succeed in breaking out of the institution, and even get to another city. Of course, the main character's friend is functionally insane and runs off as soon as they get to the city. A week later, she hears about a string of serial killings in the area, and she knows it's her former fellow patient. The story becomes centered on finding and stopping the patient while trying to cope with all the changes that have occurred just in the past year.
When she finds the patient, she discovers that the person who had killed her family and set her up to take the blame had actually been working for the insane woman. The woman tells the main character, "It was only an experiment. I could have picked anyone... but you seemed like a good test subject." Out of vengeance, the main character kills the patient, discovering that she really has gone insane... but she doesn't care. She gets caught and winds up going back to the same mental institution, accepting that that's where she belongs.