Brooke's Colorful Creations

I still don't understand why  Farquhar was trying to break the bridge.  I get he was tricked but wouldn't the soldiers want to hear him out? He was trying to do something for a good cause anyways.  Did he even get to talk and try to defend himself? Or did they just go straight to punishing? Was Ambrose Beirce trying to show how unjust the Union army was? What did Ambrose want us to take away from them doing that?

 " Peyton Fraquhar was dead; his body, with a broken neck swung gently from side to side beneath the timbers of owl creek bridge. " I was most certainly not expecting Peyton, Fraquhar to die like that,  I had no idea it was all part of his imagination.   The author described his whole journey home in such a complex way.  It simply couldn't have been a dream.   I understand the theme but what does the author want us see through this terrible scene.  Does he want to make it clear that you can be in a whole other world other than r e a l i t y                                                                                              I m a g i n a t i o n   ?