The ending ruined it for me (spoilers)

As I am one of the 3 people on Earth who actually enjoyed this movie, I was very disappointed with the ending...with her mental illness, and obsession with Sara, Rebecca was, throughout the movie, portrayed as something of a sympathetic villian, who wasn't in complete control of her actions. Thereby the cold moment, at the end, where Sara says "we were never friends" and then stabs her, felt kind of cold and out of place...especially as Sara was also quite a warm and likeable character. That whole scene could have been redeemed, if Sara had to a certain extent realized that she did feel sorry for Rebecca, reflected off the fact that they did use to be friends, and simply became teary eyed before being forced to kill her...it would have fit her character profile a lot better as well. Or perhaps if the stabbing hadn't killed her, and the movie instead ended with Sara visiting Rebecca in a mental ward, and Rebecca not having any recollection of Sara trying to kill her.