Once in a while, we know why an author writes a book because they tell us why. We know, for example, that William Faulkner wrote one of his most well known short stories, "A Rose for Emily," because he wanted to explore what might happen when a young woman's normal human desire for love and a family are thwarted by an overbearing father figure—Faulkner discussed that in an interview. In many cases, though, a writer doesn't disclose their intention. In literary criticism, the attempt to discover a writer's intention in the absence of a clear statement of intent is known as the "Intentional Fallacy," because we cannot determine with any certainty what the writer's intent is. The best we can do is to look at internal evidence and make an educated guess at intent.
We know that Michael Shaara, who wrote no historical fiction before a family trip to Gettysburg, became, according to his son Jeff Shaara, "obsessed" with the battle after he and his family visited the battlefield. We can infer, then, that the history of the battle inspired Shaara to write its story—but there is nothing in Shaara's words that directly states what he intended the novel to tell us. We can, however, look at the novel as a whole and conclude that Shaara sees the battle—and perhaps the Civil War as a whole—as a struggle of good men on both sides for belief systems that are incompatible but understandable.
One of the main characters in The Killer Angels, Joshua Chamberlain, who commanded the 20th Maine on Little Round Top in a desperate fight to maintain the Federal line against a Confederate unit, says to a group of Federal deserters he is trying to convince to re-join the fight,
Here you can be something. Here’s a place to build a home. It isn’t the land—there’s always more land. It’s the idea that we all have value, you and me, we’re worth something more than the dirt. I never saw dirt I’d die for, but I’m not asking you to come join us and fight for dirt. What we’re all fighting for, in the end, is each other.
Thanks to much research done on why men fought in the Civil War, we know that motives for fighting varied widely among troops on both sides. Chamberlain's words—from the pen of Michael Shaara—resonate with us strongly today as a reason for many troops to fight, because many troops, on both sides, were clearly not fighting to save or end slavery, but many expressed their loyalties to each other, to their state, or to the Union.
If we take The Killer Angels as a whole, the novel is not about a war to end slavery—with ill-intentioned Confederates fighting well-intentioned Federal troops—it depicts good men on each side fighting because they believe their world is worth saving. Shaara seems uninterested in defining the men who fought at Gettysburg in terms of their political views or even their belief systems. Instead, they are all men of conviction fighting, in large part, to survive and to make sure their comrades survive. In the end, I am sure that Shaara believes the existential goal of the war—freedom and equality for all men and women across the country—is a worthy goal, but his intention, I think, is to depict the struggle, not its goals. The novel may or may not be important to history—its importance lies in each reader's perception—but it is a masterful depiction of the men, events, and sacrifices in one of the most important battles of the Civil War.