Trivia: Director Paul Verhoeven approached Peter Weller for the role of Nick Curran but the producers opted for Michael Douglas instead.
Trivia: Director Paul Verhoeven actually has a PhD in mathematics. He received it back in the 1960's in Holland. He left the field and became a film director. (From Director's commentary on "Basic Instinct: The Original Director's Cut".).
Trivia: Before Sharon Stone was cast as Catherine, Michelle Pfeiffer, Julia Roberts, Kim Basinger, Kelly McGillis, Greta Scacchi, Emma Thompson and Mariel Hemingway all turned down the role.
Trivia: Another person offered Stone's role was Meg Ryan, who decided not to break her clear cut image. But 10 years later, she decided to do the erotic thriller 'In the Cut.'
Trivia: Michael Douglas allegedly wanted the role of Catherine to be played by an equally big star. He thought the erotic movie could possibly do career damage, so the leading actress should take the risk with him. He suggested Michelle Pfeiffer, but she disliked the content, later describing herself as "modest" and "kind of prudish."
Answer: No definitive answer is given in the film. Catherine didn't need a reason for anything that she did. As for Beth, that was just a bonus, or a coincidence. It could be argued that Catherine targeted Beth, and Nick was just unfortunate to be involved with Beth.
ChristmasJonesfan