Showing Wild Justice by Kelley Armstrong

Ex-cop Nadia Stafford has a dark secret. After taking the law into her own hands she was kicked off the force... and entered the shadowy world of guns for hire. She has her own strict code - works only for one crime family, only kills the really bad guys. But when a hit goes tragically wrong, Nadia is devastated. Is it time to leave the business for good?

Before she has time to decide, Nadia discovers that her own life is under threat. And worse - that terrifying events from her past may have triggered the attacks. With the help of Jack, her enigmatic mentor, Nadia is forced to upon a dark path - towards the truth and towards her final destiny.