Motives (spoilers)

She didn't know immediately. After a while it becomes obvious when he's not getting phone calls, not mentioning getting together with anyone. She's obviously done this enough times to see he was lonely and probably no one would miss him if he disappeared. They were a murdering, scamming couple who targeted people to rob by her pretending to be interested in them and him stalking them and ruining their lives. This enabled them to steal their money. He was a real psycho though which showed when he murdered the couple he drove home. She was the boss of the operation and at the end she got tired of her partner and killed him. Then she went onto another potential victim, as shown in the last scene with that young woman who was "new in town".