Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
The Killer Angels features several epigraphs:
"When men take up arms to set other men free, there is something sacred and holy in the warfare." – Woodrow Wilson
"I hate the idea of causes, and if I had to choose between betraying my country and betraying my friend, I hope I should have the guts to betray my country." – E. M. Forster
"With all my devotion to the Union and the feeling of loyalty and duty of an American citizen, I have not been able to raise my hand against my relatives, my children, my home. I have therefore resigned my commission in the Army. …" – from a letter of Robert E. Lee
"Mr. Mason: How do you justify your acts?
John Brown: I think, my friend, you are guilty of a great wrong against God and humanity—I say it without wishing to be offensive—and it would be perfectly right for anyone to interfere with you so far as to free those you willfully and wickedly hold in bondage. I do not say this insultingly.
Mr. Mason: I understand that." – from an interview with John Brown after his capture
There's also a quote at the end of the book:
"Thus ended the great American Civil War, which must upon the whole be considered the noblest and least avoidable of all the great mass conflicts of which till then there was record." –Winston Churchill, A History of the English-Speaking Peoples
So, there are four epigraphs in this book. The Forster and Lee quotes seem to align with each other, and the Wilson and Brown quotes are also of a piece. Lee didn't believe in slavery—but he identified Virginia as his homeland, the place where his family and friends were, and he couldn't fight against it, just as Forster said he couldn't choose his country over his friends.
John Brown actually tried to lead an armed rebellion against the slave states before the Civil War, and he and his son even massacred a group of slave-holders using swords. Although Woodrow Wilson—who was sympathetic to the South, in some ways, despite the quote—probably wouldn't have approved of John Brown, his argument that fighting to set men free is sacred resonates with Brown's sense of a religious mission to liberate the salves.
The Churchill quote from the end of the book also helps back up this argument—the war, in Churchill's view, was noble and unavoidable because it dealt with a crucial moral issue—slavery—and involved people fighting over principles, not just conquest and glory.