Just an OK movie and acting was just meh despite such good actors actually being cast. Enough suspense to keep me involved but I like these types of films and will give all of them a chance...if you don't like suspense films to begin with then this movie will bore you and just leave you too confused to care. Took me a while to figure out what I interpret it to be...for those looking for fellow peeps to share their thoughts here is mine....
SPOILERS BELOW
Grace is indeed the "bad guy". Lied her way to become the Nanny, has a split personality due to her abusive past, used both her personalities to seduce both Mary and her husband to get what she wanted. Although it seems unclear because what she wants is actually 2 different things. Grace just wants to be loved and a part of a real family, Margaret just wants to watch the world and everything good about family burn. BUT, I think Mary is the "bad guy" too...its true she turns into another person when she writes (the cigar smoking gives her away when shes in that mode, just like the drinking and sexuality gives grace away). She took the "lean into the dark" literally and let herself go down that rabbit hole to write again, and let herself be seduced by grace. You can see her loose grip on reality early on in the film, and as her desire for grace takes hole, she secretly suspects her friend is fooling around with her husband (and we all saw her freak out at the dinner table just at the thought of only a "dream" she had of her husband cheating, imagine what she can be driven to do if she thought it was true of her friend!) Mary killed her friend, which is why the scissors were in her purse, and she was missing for 3 hours at night, and she went in disguise...remember "that could be anyone!"..yes and it was her. At the end of the film, Grace ends up in a psych ward, and Mary ends up free...but they both have split personality disorder (also seen diagnosed by her friend, another motive to get rid of her lest her true self be exposed to her family). That is why at the end she puts the book on her friends grave saying it was "for you", she feels guilty for killing her, but somehow attributes her death to her success of being able to write the book because she let herself go to the dark place. This is also why she quickly forgives Grace, she understand her and loves her because she is like her. She visits in secret though (duh her husband would not allow his wife to visit the girl who seduced and almost killed him!). That's why she goes in disguise, but then seeing her do that proves SHE killed her friend. YIKES! this movie is about 2 crazy chicks who are perfect for each other :)