If I Had You (2006)

Shortly after the death of the father who regularly abused her, steely investigator Sharon Myers moves from the city back to the sleepy burg where she grew up. The morning after a class reunion, the body of a local teacher turns up, and all the evidence points to one Phillip Andrews, who'd had a brief affair with the victim. The only problem is, Phillip is married to Sharon's childhood best friend, Helen - and he's only just broken off yet another tempestuous affair, this one with Sharon herself. Carefully balancing her loyalty to Helen, her obsession with Phillip and the need to keep fellow detective Harry Fielding in the dark, Sharon covertly manipulates the investigation. Eventually, though, Harry and Phillip both begin to suspect that Sharon herself is the killer.