Symbolism, Imagery, Allegory
You may have already guessed this, but the "killer angels" found in the book's title are… wait for it… human beings. That's a pretty snazzy metaphor, and it was inspired partly by Hamlet—now that's a pretty hot pedigree. In Act II, Scene 2 of the play, Hamlet complains that even though human beings are great—"in action how like an angel! In apprehension how like a god!"—he can't find anything to delight him in this "quintessence of dust," so called because human beings ultimately collapse into dust when they die.
In a flashback scene in The Killer Angels, Chamberlain memorizes this speech and recites it to his father:
Once Chamberlain had a speech memorized from Shakespeare and gave it proudly, the old man listening but not looking, and Chamberlain remembered it still, "What a piece of work is man… in action how like an angel!" And the old man, grinning, had scratched his head and then said stiffly, "Well, boy, if he's an angel, he's sure a murderin' angel." And Chamberlain had gone on to school to make an oration on the subject: Man, the Killer Angel. And when the old man heard about it he was very proud, and Chamberlain felt very good remembering it. (2.4.28)
Chamberlain thinks that human beings are killer angels because, while he believes they are made in the image of God, they also are broken, fallen into violence and depravity. The Battle of Gettysburg is a good example of this: the soldiers act heroically and for causes they believe in while at the same time killing each other. The best and worst that humanity has to offer are on display, sometimes mixed up in one and the same action.