
Continuity mistake: When Campbell is in the basement, he drops a book of matches. They fall all over the floor, but in the next shot they change their position and lay in a neat little formation. (00:55:30)
Continuity mistake: In the beginning, Katherine Heigl is working on a cadaver and has surgical gloves on. When she thinks someone is after her, in the close up of her squeezing the surgical knife, the gloves are gone.
Answer: He doesn't kill her because she was the only nice one, whereas her friends( Paige, Lily, etc.) were all mean to him at the beginning.
cameron_eats_lemons