What is the theme in "Horseman in the Sky" by Ambrose Bierce?

The theme of "Horseman in the Sky" by Ambrose Bierce is the emotional pain caused by war. In the story, a soldier has to decide whether to shoot or not.

A young Union soldier fell asleep when he was supposed to be standing sentry, only to wake up and realize a Confederate soldier had discovered his troop. He prepared to shoot when then other soldier looked up, and then the Union soldier struggled with the decision. Finally, he shot the other soldier's horse, and the horse and Confederate soldier went over a cliff. It turned out the Confederate soldier was the Union soldier's father, and that was the source of the Union soldier's emotional pain.