This is not merely a murder story. It is not merely a mystery. Rather this book is a deep dive into modern Chinese culture, especially life of average people in small town, small city China - something that is pretty much never shown in the West.
At the heart of this book is the question of nature and nurture - what makes someone a bad person? What makes a child become bad? And how does Chinese society view children? To understand this book you should understand a few things:
1. China has experienced capitalism out of control since the 90s. What does that mean? On the surface, the gleaming megatropolises of Shanghai, Beijing, Shenzhen etc. built on the backs of undocumented laborers and masses of rural poor in sweatshops and factories, but between that, in little town China, in the little lives of ordinary townspeople it has created broken marriages as desperate young women do anything to "marry up", feelings of inadequacy between people from town and city backgrounds, and enormous societal pressure on children to achieve in schooling. The latter, the emphasis on schooling is all important in this book as it is in modern China. A good student who goes to a good school, gets a good urban job, and most importantly, gets a big city residency (this is the “documentation” that makes one no longer a migrant), can change the path of a rural family for all future generations. As a result, a good student is almost synonymous with a good child. This is a very important theme for the book.
2. Communist China was not an equal society. Instead your family background defined your social standing, marriage prospects, work unit assignment, and opportunities. If you had a bad background (i.e. if you or people in your family had bad politics, were wealthy in the past, were landowners, were intellectuals), you had essentially a black mark on your name that could not be easily erased. In modern day China, that has again been turned on its head, with capitalism running rampant and money treated as king. But there is still very much an underlying current of "sins of the father" and "family shame" in small town culture. This plays a huge role in how the characters in this novel see their lives, see their opportunities to change things around. It also plays a big role on how characters see each other and themselves.
3. In modern day China, the censors for art, literature, and media are extreme - material that openly suggests a subversion of the justice system, of the fabric of society is censored. But cleverly written books that comment on social and political corruption etc. are able to get through the censors so long as justice is served in the end. In this atmosphere, this book is more interesting in what it leaves ambiguous and unsaid.
4. This is the last of a trilogy following a police detective and professor of criminology called Yan Liang in various points of his life. You do not need to read the prior books to understand this one (each are distinct mysteries). The only linking thread is that each book has Yan Liang at a different point in his life as a detective, then professor, then now, in Bad Kids, as a retiree. His perspective of crime changes over the decades as he sees the costs and injustices in society and in the justice system. In an earlier book, Yan Liang watches a group of people sacrifice their career, families, lives to bring justice to an old crime while suffering the neutrality of an unforgiving justice system. He becomes jaded and cynical. He does a few things "not by the book." Knowing this you may have more flavor in understanding Yan Liang's thoughts and actions at this latter point in his life.
5. The entire trilogy is on IQiYi as part of their "Light On" series. This book has been turned into an excellent TV show called "The Bad Kids" or "Hidden Corners." Check it out, I believe it has good subtitles