The issue of slavery does not seem to have much relevance to most the main characters at the Battle of Gettysburg in The Killer Angels, aside from the passionate anti-slavery views held by one of the novel's main figures, Colonel Joshua Chamberlain. Slavery is not foremost on the minds of the main Confederate characters. General Robert E. Lee released his own slaves (inherited from his wife, the step-daughter of George Washington) at the beginning of the war; he, like most of the other primary Southern figures, were fighting for states' rights--in Lee's case, Virginia. (It should also be pointed out that most of the main characters in the novel are Confederates, not Federal soldiers. Shaara tells the story from a mostly Southern point of view.)
Chamberlain, however, believes it is a new type of war.
"Some of us volunteered to fight for the Union... Many of us came... because it was the right thing to do. All of us have seen men die. Many of us never saw a black man back home. We think on that, too. But freedom... is not just a word."
Chamberlain lectures his men that no war has ever been fought "to set other men free."
"... this hasn't happened much in the history of the world... This is free ground... No man has to bow..."
For Chamberlain, it is not about "dirt" or the land. "There's always more land." The war is being fought to assure the freedom for all men--black and white.