It's more of the 'castle' tv series. It shows all the people who could be involved and as time goes by, you're suspecting someone else but last 10mins, you get a new clue and the killer is someone else.
I will not lie, I actually did consider the fact that Tristan wasn't Mr. And Mrs. Crest's child. I even went through some chapters to see if anything about him was defined, like physical description that could link him to either Alexandra or if anything could say that he's got the blue eyes and Mrs. Crest does too but not Mr. Crest of vice versa. Although I know where is author is coming from in the whole 'the dads the killer' chapter but I feel like it could've been better. His killing is more based on last moment anger and paranoia that his children will leave him. It's more psychological like in 'stalker' tv series. (I know, I watch a lot of detective shows, I'm kind of a pro now xD) but in that case, she should've shown through the chapters that the reason was more based on psychological distress than physical activity.