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: There's no definitive answer to this and the ending is deliberately ambiguous and open to interpretation. The audience is left to speculate whether or not Catherine kills Nick, or if she intended to kill him but changes her mind because she loves him, or intends to kill him at a much later time, and so on.
raywest ★