Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
"The unexamined life is not worth living." —Socrates, Plato's Apology
What's up with the epigraph?
This whole book is about Shaka learning to live the examined life. It's an unwillingness to examine his life and deal with his emotional problems and tough experiences that pushes Shaka into the extreme state of mind where he's willing to shoot someone.
And it's the long, slow process of learning to look at his life, understand it, and try to change it that helps transform him and redeem his past. So this is a great quote to use.
On top of that, as a reference to one of the most widely read philosophers of all time, it foreshadows the importance of reading philosophy and trying to learn wisdom that Shaka will discover in the book.