Warning: this question consists entirely of major spoilers
At the end of the movie, it was revealed that Simon and Jacqueline plotted together to kill Linnet. However, when my family watched it, they couldn't agree on when or why exactly Simon and Jacqueline planned the murder.
One theory is that they made the plan before Simon met Linnet. Simon pretended to fall for Linnet so he could marry her, kill her and take her money to live wealthily with Jacqueline.
- The issue with this theory is that Jacqueline seemed genuinely distressed while watching Simon and Linnet dance, and she wouldn't have had a reason to feign distress, since she didn't know that Poirot was watching her. Although it's possible that it hurt her to watch Simon woo Linnet even though she knew he was pretending.
The other theory is that Simon really fell for Linnet, but as he saw Jacqueline throughout the honeymoon, he realized that he still loved her, and he made a plan with her to get rid of Linnet so they could be together again.
- This theory makes less sense to me, but my brother pointed out that the theme of the movie is "people will kill for love," and this theory fits that theme better (whereas the first theory is more about killing for money). Also, it would explain why Jacqueline looked distressed while watching the other two dance.
Is there any information in the novel or from statements by the director or screenwriter that indicate when Simon and Jacqueline made their plan? Or maybe there was a detail in the movie I missed that answers this question?