How is the issue of slavery dealt with by the characters in The Killer Angels?
The main concentration of Michael Shara's Pulitzer Prize winning book The Killer Angels is on the development of the battle situation at Gettysburg in 1863, and how that plays out in the first part of July to in part determine the course of the war. So slavery isn't a major theme dealt with in the novel, but it is mentioned at least twice that I recall, both times by brothers Thomas and Joshua Lawrence Chamberlain. Joshua is the commanding Colonel of the 20th Maine Regiment, new to the battlefield and decisive on Little Round Top. His younger brother Tom is his Lieutenant.
At one point Colonel Chamberlain is having a conversation with his Irish Sergeant, Kilrain, where he remarks about their being no difference between the two races, both being humans and sharing the same spark of life (paraphrasing here). At another point in the novel, after a battle the 20th is involved in which the field is strewn with Confederate corpses, Tom mentions how he can't believe that people would fight so hard to defend an institution as evil as slavery.
But slavery is mostly the afterthought to these characters. Much more pressing and present in the book was their task at hand, and how to accomplish it on each side.