What is the irony at the end of "A Horseman in the Sky"?

In the short story "A Horseman in the Sky," Carter Druse is able to override his aversion toward killing the Confederate scout by recalling the advice of his father. The irony is that the scout turns out to be Carter Druse's father.

The irony of the act is intensified when it is revealed at the end of the story that Carter Druse in fact knew that the Confederate scout was his father before he pulled the trigger. The author, Ambrose Bierce, uses the story to comment on the irrationality of war and to illustrate the effect that the Civil War had on soldiers.