Why didn't she call the cops?

Actually she would be considered murderer by the law because the crime scene was a mess with blood all over the place, would be impossible determine who do what at leats trough the blood pattern, so you can say "she can tell when she was call by Lucie and stablish a time frame that prove she dint kill those people because she wasn't in the house in the moment of the murders" stablish a moment of dead is not a precise science i meant you can't determine for the minute when someone dies and while more time past more dificult is determine with precission a hour of death and Lucie call her inmediatly after the murders she probably was close and get fast to the house and not only that but stay too much time in there to the point the aproximate time of murder and the moment when she get to the house is almost the same now is impossible for a forensic examiner determine she wasn't in the house at the moment of the murders and here become the problems for her because she move the bodys all over the house compromising the blood pattern in the floor and the original position of the body that could help the forensic to determine in what order the family was murder and where exactly every body was kill and maybe with that they would get to the conclussion was the work of just one shooter

She was doomed in the moment she move the bodies and more yet when she choose stay that much in that house but anyway i can be wrong im not an expert im just a regular guy who watch too much CSI

PD: in the begining i tough this girl was dumber than a rock but them i understand this is the hold point of the history she is someone who is ready to sacrifice her well being and future just to help someone else she have a deep instint of sacrifice taking care of her friend trough the childhood and them staying in the house when find out the other victim her first instinc was nurture her and consoling her like a nurse would do she is the kind of person who thinks in others before herself

I spent the whole movie asking the same thing. Even before she found the girl. As soon as Lucy was dead I would have been calling people!

Because she had a crush on Lucie.