At one point, Joshua Chamberlain remembers reciting a speech from Hamlet to his father, in which Hamlet states that man is "in action how like an angel!" Hearing this, Chamberlain's father says that if man is an angel, he's most definitely a "murderin' angel." Chamberlain remembers his father's words and later gives a speech called "Man: The Killer Angel."
Toward the end of the book, the term "killer angels" is used again, as Chamberlain reflects on the battle that has just concluded: "He was thinking of Kilrain: no divine spark. Animal meat: the Killer Angels" (4.6.18). This term serves as a metaphor for the goodness and the wickedness of human beings—their capacity to do something righteous, like standing up for the rights of slaves or defending a friend, as well as their capacity to cause enormous suffering and destruction.
The conflict between the dual nature of humans—they're both good and bad—lies at the heart of this novel and gives it depth. The battle isn't just on the ground in Gettysburg; it's also in the hearts of each of the novel's characters.