Why was the occurrence of owl creek written?

Answer 1

Ambrose Bierce was a professional writer, so a major reason for all of his stories was to make money.

That said: when An occurrenence first appeared in 1891 the memory of the Civil War was still fresh, and it was decidedly provocative to write a story where the Confederate cause is presented favourably and the Union is demonised. Bierce compounds the challenge by referring directly to his protagonist's slaves, and making clear that Peyton Farqhuar status as a slaveowner is what drives his patriotism. Many of Bierce' original readers would have been pro-abolition, a few would even have been black; - they would have found such notions difficult.

Bierce himself was a native of staunchly Union Ohio, but there is a common theme to many of his stories that individual characteristics are more essential, and more admirable, than social duties. Peyton Farquhar is a man with strong ideas which most liberals in 1891 would have found difficult to countenance, but his virtues make sense within his own universe, and he is validated by that.