You're all wrong. *spoiler*
I've read many threads about Katie's character and why she killed Embry and the detective. (although initially came to the forum to find out what he noticed in the video.)
Anyway...
Yes, she killed Embry because she didn't want to be abandoned. Then she forgot that she did because of the trauma.
But why did she kill the detective? He was not about to abandon her. SHE was the one who said, "I can't go with you to New Hampshire, can I". Why? because she just realized that she's a murderer and who would take a murderer? He said "We'll get you help." Because he knows she's psycho and he's going to turn her in. So she kills him to COVER HER TRACKS. Just think about what would happen to her and her cush new job if she hadn't.
Also, I don't believe that she had schizophrenia or multiple personalities. She was suffering from hallucinations that was a combination of the stress and her buried memories trying to emerge. And might have also been a side effect of her medication since her hallucinations also became stronger AFTER she was given medication by her doctor.
This means that by the end of the movie she has the knowledge of both her killings. When her friend asked what happened to the detective, she says quite happily and knowingly that maybe he's in NH. If she really thought that he never showed up to pick her up, she probably would have been a bit sad or bitter.
To me this movie was about the making of a serial killer.
If you enjoyed this movie, I recommend Taking Lives.