Trigger Finger (2012)

Morgan Westin is a teenage wallflower -- who may or may not be responsible for her step-sister's murder. Detective Harold Casey is brought on to the case once suspicion for the crime falls on Joseph, a local drug dealer with whom Casey has a long-unsettled score. When Casey discovers that the killer might be Morgan, not Joseph, he decides to help her get away with murder, so that the "right man" can end up in jail. But is Morgan a cold-blooded killer, an innocent victim, or something in between?