Perfect Stranger (2007)

(SPOILER ALERT: DO NOT READ THIS POST IF YOU DON'T WANT TO KNOW WHO DID IT)

[Now for the first mini Plot Buster, where only enough of the plot is described in order to understand the ending. Not surprisingly this resulted from a query from someone who missed a vital moment near the end of the movie, and therefore couldn't understant how things had managed to work out just the way they did.]

Perfect Stranger (2007) is a psychological thriller film directed by James Foley, starring Halle Berry and Bruce Willis.

The Halle Berry character is the murderer.

She killed her "friend" played by Nicki Aycox, because she was being blackmailed by her, after Aycox witnessed Berry and her mother killing her abusive father as a child.

Because of his broken-off affair with Aycox, Berry was able to frame the Bruce Willis character and so hide her own guilt.

But Berry ends up killing Willis in order to maintain the secret, later claiming to the police that she was acting in self-defence, as Willis must have indeed been the true murderer.

Unfortunately for Berry, her cold-blooded murder of the innocent Willis is also witnessed. Time for a sequel!

I hope that this has unraveled the somewhat tortuous plot twists for you.

Sources:
http://en.wikipedia.org/wiki/Perfect_Stranger_%28film%29