Revealing mistake: When Bill is stabbed, the pad beneath his shirt - where the knife is stabbed - protrudes.
Visible crew/equipment: When the journalist leaves the premises and Jessica steps back inside the house, the camera rises to obtain an aerial view of the property. During this upward movement, the camera and crane operator—wearing a white hat and a red-and-blue coat—are reflected in the glass panel above the door.
Answer: In a flashback scene, it was shown that he had a violent temper, so it stands to reason that he finally killed her in a fit of rage.