"B has harmed her"
When Rachel and the police investigate Megan's death, they first turn to Scott, her husband. Rachel notes to herself that she may at first be the only one who knows there is another suspect, the "boyfriend" she has seen Megan kissing in the garden - Kamal. However, when she writes herself a list of possible explanations for Megan's disappearance, including the non-specific "B has harmed her" (in which B stands for boyfriend), she does not realize that this really is the case, but with Tom rather than Kamal.
Tom
As is often the case in mystery novels, there is dramatic irony throughout the novel as characters say and think things that have greater significance or explicitly contradict the truth once the reader discovers the true culprit, in this case Megan's killer. The most significant irony surrounding Tom's murder of Megan is the degree to which Rachel and especially Anna trust him. For example, Anna thinks to herself that after finding out Megan killed her child, she is somewhat glad that Megan is dead so that she can never touch her child again, adding that she will never let anyone else care for her child. However, Tom, a more coldblooded murderer than Megan, who accidentally killed her child, is allowed near Evie even in the last scene when Anna has discovered his true self.
Anna's Comment on Scott and Rachel
Anna reports the suspicious activity between Rachel and Scott to the police; however, she thinks to herself that there is no way there is a romantic or sexual relationship between Rachel and Scott because she believes Rachel such a step down from Megan in looks and stability. However, Anna has this thought when the reader knows that Rachel and Scott have already had sex, though the incident was not a positive one for either participant.
Jess/Megan and Jason/Scott
Before Rachel reads about Megan and meets Scott, she fantasizes about their married life together, adding a good deal of romantic imagination to what she witnesses through the train window. A sense of irony is created when the reader soon takes Megan's perspective, learning about the imperfections of her marriage to Scott, especially his tendency to be overprotective.
