A Goodreads user asked:
What was the motive for Scotty killing Beth ?
This answer contains spoilers… (view spoiler)[At the end, Beth's "ghost" says that Scotty told Beth that she hated Beth's life and was angry that she chose to cheat on her husband while Scotty was working hard to make her marriage work. I honestly feel like this was the most uninteresting reason. The author takes you through so many twists to keep you guessing who it is that to leave you on that boring motive is just... disappointing. (hide spoiler)]