The main question of The Secret Keeper is the question of malice and evil intent. Everyone suffers, that's true, but not everyone suffers equally. For instance, Dorothy and Vivien experienced extreme forms of suffering including violence, death of their families, and a firsthand experience of helplessness in warfare. This leads to a confusing system of insane little behaviors, the main one being the one Laurel remembers, the murder of the stranger when she was only 16 which she saw from a treehouse.
It turns out that Laurel's curiosity's about her mom were actually not serious enough. Her identity as "Dorothy" seems less likely now, given Jimmy's observation that it's actually Vivien in the picture. Therefore, Vivien is likely a murderer who lost her mind, killed the real Dorothy, assumed her name and identity and murdered someone who was going to ruin the secret.
In this way, secrecy returns as a fundamental idea in the novel. The variable of the secret leads Laurel to investigate in many clever ways, but the fundamental secret is that Laurel has been raised by a serial killer and she never found out. Therefore, the novel functions as a quiet sort of horror story, just to show how little we can really know about people we assume to know so well.
