What is a good song that relates to The Killer Angels?

The Killer Angels was actually made into a film called Gettysburg, starring Jeff Daniels, which released a number of soundtracks. The first album, Gettysburg, was the music from the original soundtrack by Randy Edelburg. The second one, More Songs and Music from Gettysburg, included tracks by Edelburg and songs dating back to the Civil War period, including the "Bonnie Blue Flag," "Dixie," "Amazing Grace," and "Yankee Doodle." John Durant and Sandy Mitchell play American folk tunes, such as "Do They Miss Me at Home," and the American Brass Band, under the direction of John Hannan, plays more patriotic songs, such as "Bonnie Blue Flag," a Confederate marching song.
The rest of the soundtrack can be viewed by clicking on the link below. The album can be listened to on Amazon.
https://www.discogs.com/Various-More-Songs-And-Music-From-Gettysburg/release/4133453

Two songs dating to the period are logical choices here.
One of the songs most commonly associated with the Civil War has two names, "Battle Cry of Freedom" and "Rally 'Round the Flag." George Frederick Root, a professional songwriter, wrote it during the war, in 1862. He was responding directly to President Abraham Lincoln’s call for Union troops to quash the armed uprisings in Virginia. Quickly gaining in popularity, it became the song that Union Army bands most frequently played.
Another likely choice is "The Battle Hymn of the Republic," generally credited to Julia Ward Howe. While she wrote most of the war-related lyrics, William Steffe had written the tune and different lyrics around 1856. It had become popular as a church-related song, often sung at black Methodist churches. By the onset of war, it was already well known.
https://www.britannica.com/topic/Remembering-the-American-Civil-War-1763580/Fighting-the-war