Pretend You Don't See Her question

I don't get the ending of this book??? My understanding is that someone hired a hit man to kill both the daughter and the mother. Lacey saw the hit man when he came to the deceased daughter's apartment and killed the mother. I'm confused...who hired this him man?? I know why the mother was killed, because she had figured out that her daughter was murdered. Who wanted the daughter and mother dead? Who hired the hit man??