Continuity mistake: Douglas opens a mailbox to check the mail for clues. He then puts the mail back and closes the box, a second later you hear him closing the mailbox again.
Continuity mistake: When Roxy runs off the bridge you see the Lotus crashing over with its low beams on - when you see the car fly off the bridge its brights are on.
Continuity mistake: In the sequence where Michael Douglas is driving along the coastal highway, his car's center rear-view mirror appears and disappears over and over.
Continuity mistake: When Roxy tries to run over Nick the left front light is broken off - in the next scene it is back on.
Continuity mistake: When the detectives ask Catherine to go in for questioning, she has her hair down. Then, a minute later, she comes out with her hair up. Problems: one, there's no way Catherine could do that on her own, and no-one else is in the house; two, even if she could it would take much more than a minute, say thirty minutes.
Continuity mistake: When Nick and Gus are waiting for Catherine to get changed, out of the corner of his eye, Nick sees Catherine step, naked, into a white dress, then, again, she steps into the same dress.
Continuity mistake: When Michael Douglas and his partner are in the diner, the cap on the Tabasco sauce bottle is off. A second later, it is on.
Continuity mistake: In the scene where Rocky and Nick are talking in the bathroom you can see Nick's back in the mirror - he has two nasty looking scratched up areas of bare skin on his back - then when he turns and returns to the bedroom you can see that he now only has a couple of superficial marks.
Continuity mistake: At the beach house, Catherine is cutting up a large piece of ice with an ice pick. Depending on the direction of the camera, the ice changes size: in one shot it's all together; but in the next shot there's a large chunk pushed to the left of her.
Continuity mistake: During the first sex scene between Michael Douglas and Sharon Stone, Miss Stone's nipples at first are very erect, but in the next shot are not.
Continuity mistake: When Nick and Gus are driving Katherine to the police station for the first time, they drive past the same stretch of coast about three times throughout the scene. Most obviously seen by the position of the small island - it jumps from being in the distance, to being even with the car, then behind the car, then in front again etc.
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