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."
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: Right at the start of the film, look carefully at the pigmentation around her nipples, then look at the breasts of the women. You can tell who the killer is straight away.
Answer: She is portrayed as a psychopath with an uncontrollable urge to kill.
raywest ★
I wouldn't say that she had an "urge" to kill. Rather, she was willing to "dispose" of people by killing them. She would kill people who were in her way or were no longer interesting/useful.