The term "Killer Angels" is actually attributed to one of the central characters, Union Colonel Joshua Chamberlain. According to Chamberlain, he had recited a Shakespearean piece that included the quote,
"What a piece of work is man... in action how like an angel!" (Hamlet, Act II, Scene 2)
When Chamberlain's father heard it, he remarked,
"Well, boy, if he's an angel, he's sure a murderin' angel."
The junior Chamberlain later made an oration on the topic, entitling it "Man: The Killer Angel," of which his father was very proud.
The recurring metaphor of the title is seen early on when General Buford spots an angel with uplifted arms in the Gettysburg cemetery. After the battle, it has disappeared, as if it has flown away after being witness to so much human carnage. I believe the title is meant to show the complexities of man: How man is created in God's image, and how man can serve and care for others at one moment and become a murdering savage the next. Chamberlain relates how the Union is fighting to free the slaves but they must kill others in order to do it.
Many of the main characters are highly religious. Chamberlain is obviously a devout Christian as are many of the enlisted men on both North and South. Several of the Confederate leaders are as well, especially Robert E. Lee and J. E. B. Stuart. (The late "Stonewall" Jackson, who was killed just weeks before the Battle of Gettysburg, was an extremely pious Christian who often fought on Sundays because he believed God favored him when he did so.)