This novel is a dark story with high stakes. Before long, the reader realizes that the protagonist is actually the antagonist, and although at first, the reader might identify with the orphan's journey of discovering his identity and cultural heritage, he goes too far and starts murdering innocent people for being white. The serial killer, or Indian Killer, believes a narrative about what it means to be Native that is actually untrue and damaging, but it is the narrative that people have given him about his own culture.
That means that the white people in his life have passed along the idea that what it means to be properly "Native American" is to murder innocent white people. That comes to bite them when the kid turns out to be a sociopath who just wants to live in the identity of his ethnic culture. He believes the narrative that racists have given him and just adopts that personality. Before long, he is a full-blown serial killer.
Another way to read the novel is that a kid wanted to be white for a long time, but people kept treating him with hostility and contempt for not being "actually white." Then he goes crazy and starts murdering white people to punish them for their racist mistreatment of him. In either interpretation, the analysis is the same: racism of any kind is damaging to society because it brings horror and mistreatment to a community—even death, as this novel (as well as history) well indicates.
