If Vera had not been so emotionally unstable at the book's end, she might have been able to spot the killer in her room, somehow trap him until the authorities came, and the mystery would've been solved. But Agatha Christie's publisher would have seen potential in this and demanded she write yet another detective series as a spin-off. Agatha foresaw this in the early stages of planning the novel and dreaded the idea of being committed to yet another detective she couldn't care less about as she already had enough on her hands (Hercule Poirot, Miss Marple, Col. Race, etc). Thus, she made Vera's personality less stable than originally planned and wrote in her suicide to kill any possibility of having another detective on her hands.