A Kiss Can Be Even Deadlier If You Mean It
At the end of the book, Rachel learns that she misinterpreted the kiss between Megan and Kamal—it's not Kamal she should have been looking out for; it's her ex-husband, Tom. See, Megan was pregnant with Tom's baby and Tom killed her. Rachel misinterpreting the kiss between Megan and Kamal doesn't really change anything, though. Even if it hadn't happened, Megan would still be dead.
When Rachel realizes Tom is the Bad Guy (yeah, capital letters), instead of going to the police, who wouldn't believe her anyway, she goes to get Anna out of the house. It's a strange bit of compassion for her former rival. However, even though Anna knows Tom is an adulterer, she can't quite believe he's a murderer. Rachel eventually convinces her that Tom killed Megan, Tom comes home, there's a fight, and Rachel kills Tom with "the vicious twist" (38.16) of a corkscrew. Whoa.
An extra twist (literally) comes courtesy of Anna, who wants to make sure Tom is dead. Not because he's a killer, but because he lied to her. That's Anna for you; she's petty to the end.
So the mystery is solved, but what does it all mean? Well, Rachel realizes that in her marriage to Tom, "[her] life with him—was an illusion" (38.8). It's traumatic at first, but the revelation helps her move on with her life. When she's in Tom's house, she watches the train goes past, and she sees "travelers warm and safe on their way home" (38.17). Previously on the outside looking in, now Rachel is on the inside and she wants to get the heck out of there.
And get the heck out is exactly what she does. She has her last drink with Cathy, then sobers up—hooray—and takes a nice vacation to gather her thoughts. At the book's last line she says, "I have to get up early tomorrow morning to catch the train" (38.46). But where is she going? We don't know, but we hope she's finally going to stop going back and forth and instead move forward.