The Perfect Marriage (2006)

Annie Grayson murdered her Californian husband Martin to collect his $250,000 insurance, shared with her lover Brent Richter, who provided poison and idea. But it's squandered, they split up and she moves to Philadelphia as Marrianne Danforth. Three years later she's remarried to real estate tycoon Donald Danforth's son Richard and their charity executive. The next year Brent tracks her down and sees trough her claim to be happy. They plan to murder dad in law first, which succeeds, then Richard. Devoted secretary Carrie Hollings gets wind of Marianne's lies, so she's killed by Brent, but the suspicion doesn't die with her.