Poor Cecilia. She escapes her abusive boyfriend, only for him to commit suicide, and then it turns out he is actually still around, stalking and torturing her using an invisibility suit. That means that no matter where she goes, Cecilia will never be safe and can never lead a normal life without Adrian lurking somewhere around, ready to pounce on her the moment she lets her guard down. All because she left him.
Then Adrian manages to get all of Cecilia's friends to turn on her, isolating her even more. And when she tries to reconcile with her sister? Adrian slits her throat and plants the knife on Cecilia, framing her for the murder and getting her thrown into a mental hospital. Cut the poor girl a break...
And even when he is revealed to be alive to the public, he spins things around so that he isn't to blame. No wonder Cecilia decides to kill him.
Oh, and Cecilia finds out she's pregnant with Adrian's kid. So now she has to worry about herself AND her unborn child.
Emily's fate. She helps her sister escape her evil boyfriend, only to get a vicious email attacking her from the same person, who claims it's not her. And then when you decide to hear your sister's side of the story, you suddenly get your throat slit. Emily's dying thought might've been that her sister hated her...
Even if you don't like how James was so skeptical of Cecilia's claims throughout the film, it does hurt to see him and his daughter getting beat up in their own home. Sydney is James' top priority, so being unable to protect her must have been devastating to him.
The ending. While Cecilia is free at last from Adrian's control, she has hardened big time, even showing signs of sociopathy like Adrian. She will never be the same person she once was. Also, her sister is still dead and her relationship with her closest friend is likely damaged.