Megan appears to be the book's most complicated character, both because we spend the least amount of time in her point of view out of our three main characters and because the other two characters have such conflicting views of her.
When we first see Megan, she's "Jess," the fantasy alter-ego Rachel foists upon her. Rachel thinks Jess is perfect—a model wife, a glamorous individual. But the more we learn about Megan, the more we realize she isn't perfect at all. She says so herself in this line, where she reveals her marriage is also far from perfect:
I am not a model wife. I can't be. No matter how much I love him, it won't be enough. (4.7)
Thing is, the more we learn about Megan, the worse she seems. She's cheating on her husband. She hates babies. As Rachel says, "I didn't know her, and [now] I'm not sure that I like her anymore" (21.82). Megan, in other words, is no Jess.
But then we start to see similarities among Megan, Rachel, and Anna. Megan and Rachel's nosiness gets them into bad situations. After Megan sees Anna yelling at Rachel in the garden, she gets the idea to babysit for Anna in order to find out more about her life. "I think I got the idea after I heard her yelling out in the garden and I wanted to know what was going on" (2.17), she admits. This leads to Megan having an affair with Anna's husband, Tom. Megan says she is a "mistress of self-reinvention" (2.24), but now, like Anna, it seems she's really just a mistress.
So Rachel has built Megan up, and Megan has torn herself down. Now we get to dig in and get to the roots of her problems. She hates babies because she accidentally killed her child, Libby, when she fell asleep in the bathtub. That'll have lasting effects. And she has abandonment issues because her first boyfriend, Mac, left her and never came back. Plus her brother, Ben, died in an accident when they were teenagers. Nothing good has ever happened to Megan.
Megan has a tendency to run from uncomfortable situations, but she's also co-dependent. This internal conflict leads her to make many of the stupid decisions she makes, like cheating on her husband and getting pregnant with another man's baby.
Having an affair is one of the few times Megan feels in control of her own life. "That's the thing I like most about it, having power over someone. That's the intoxicating thing" (4.14). In her marriage, Scott has the power—he's a controlling spouse, reading her e-mails—but in an affair, Megan has the power.
Sadly, Megan ends up getting killed when she finally does the right thing. She decides not to seduce her therapist (which is hard because he's so helpful and sexy), but she does take his advice: "I can't help the way I am. 'You can help what you do, though.' That's what Kamal says" (33.5-33.6). She then thinks, "I'm not sure if I can remake myself as a good wife, but a good mother—that I have to try" (33.22). So she tells her husband she had an affair, and tells Tom she's pregnant with his baby.
Like Rachel, Megan wants to reinvent herself. Unfortunately, the controlling men take over in the end: Scott abuses Megan, so she runs to Tom, who kills her. Poor Megan is a victim for most of her life. Other than going back in time and undoing her affair, we're not sure what she could have done differently to save herself.