Recently listened to a book, The Wicked Girls, that to me is clearly based on the real-life story of the novelist Anne Perry, who along with another girl, killed a woman while a teen (also the basis for the movie Heavenly Creatures). Perry's story, assuming she hasn't killed anyone as an adult (which seems a safe assumption to me), gives rise to questions about the nature of evil, cruelty, and rehabilitation. Some of these same questions are tackled by this thriller, which is about two women who meet again, twenty-five years after committing and being prosecuted for a murder as eleven year old girls, and find themselves entangled in a serial killer's rampage. To be honest, I found myself wavering considerably on this book. To me the whole thing hinged on the construction of the murder that they committed as kids, and this is precisely the issue that the novel spends most of its length trying to obfuscate. What makes Perry's case so disturbing and interesting is that the murder she committed was quite premeditated. Her friend's mother was going to take her friend away, so they killed the mother in order to stay together. The killing was not quick or simple; it required twenty whacks on the head with a brick. And now the person who committed this crime is free, and she walks around as easily as you or me, writing books, giving interviews, and living a very normal and, to all appearances, quite matronly existence. That is fascinating. The story told in this book is much less so.
However, I understand why Marwood wrote it this way. This is the third book I've read recently which featured a character who had acts that the reader was meant to think are vile or morally gray. In one of those books (unnamed because this is a spoiler), it turns out that the murderer actually killed another guy in order to stop him from raping and killing a girl in a war-zone. And in Jeff Zentner's Goodbye Days, a kid is ostracized because he sends a text message and his friend's attempt to reply, while driving, result in him crashing and all the passengers in the car dying (note, the protagonist of this book isn't in the car, he's just a guy, somewhere else, who sent a text message). In both of these cases, the act is so far from being morally ambiguous that I threw up my hands in frustration. Like, we all agree that killing in self-defense or defense of another is okay, but if you want it to be even _more_ okay, then surely it's alright in a warzone, where there is no law, and where your victim is a soldier who is abusing his authority. Similarly, there is nothing wrong about sending a text to someone who is driving. If there's any culpability, it's in the person who engages in texting while driving, not the person they're texting with.
The problem, however, is that if these books were written in a way that was actually morally ambiguous, they would've been taking something of a risk. In Zentner's book, the obvious solution would be if the protagonist had been the driver of the car and if he'd been the only person to survive. Texting while driving is not good, but it's also something lots of people do, and yet it's only when you crash that suddenly you're a murderer. That would be a classic examination of moral hazard and of hypocrisy. But if that'd been the story, people would've hated the protagonist, and they would not have enjoyed the book. Which is absurd, because literally three quarters of people have texted while driving. It's sort of an Emperor's New Clothes situation (similar to, say, underage drinking or using illegal drugs or cheating), wherein a massive percentage of the population is doing something--if it's not you, then it's your father, your mother, your kids, or your husband--and yet we pretend it's somehow beyond the pale.
I don't think it's impossible for a book to succeed commercially if it contains ambiguous morality. I mean, it's especially true when we have thrillers. Gone Girl contained some terrible people. The protagonist of The Girl On The Train was a terrible and terribly self-absorbed alcoholic. But in general, and this is entirely my own unscientific impression, it seems that the authors of most commercial hits have tended to play it safe when it comes to moral questions.