Why do you think bierce, a former union soldier, chose to tell his story from the point of view of a confederate supporter?

It was basically a criticism of romanticism. The hero is a helpless romantic who sees something noble and romantic about the war and the confederacy even though in reality the confederacy were a bunch of racist slave owners. This is done to show how the romantic mindset can alter what's really going on and he was a realist and wanted to show that it was a bad thing.