Character mistake: When Beth encounters Nick after Gus has been murdered, she puts her hand in her pocket and steps forward, prompting Nick to order her to freeze and to take her hand out of her pocket while aiming his gun at her. However, she just keeps reaching for what's in her pocket and approaching Nick, resulting in him shooting her. Considering that she works on the police force, she absolutely should have known for a fact that she was being suspected of reaching for a gun, but thinks nothing of it.
Character mistake: When we see the door to Elisabeth's, her name is spelled with a s. Later when Nick looks her up on the computer, it's spelled Elizabeth.
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