This scene is a subtle one in Part II of the story, which is a flashback. When the scout, dressed like a Confederate soldier, stops at the Farquhar plantation, Peyton asks for news about the front lines of the war. The scout tells him that the Northern forces have secured Owl Creek bridge (a railroad trestle) and built a stockade, suggesting that the site will be a staging area for further invasion into Southern territory. The scout continues, giving Peyton this important information:
The commandant has issued an order, which is posted everywhere, declaring that any civilian caught interfering with the railroad, its bridges, tunnels or trains will be summarily hanged. I saw the order.
This is clearly a warning. It is Peyton who continues the conversation, believing that he is speaking to a friend and ally in the war. He asks about how well the bridge is guarded. Only two guards, he is told. He then asks, with a smile, what could be accomplished if one of the bridge's guards were evaded and the other overpowered; clearly, Peyton is thinking about sabotaging the bridge, despite what he has just been told about the penalty for such an action.
At this point, the scout pauses and thinks before answering. Having made a decision, he tells Peyton the bridge would burn easily. Why does he answer in this way? He could have told Peyton at the beginning of their conversation that the bridge was well guarded, and then he could have told him that nothing could be done to destroy it. In other words, the scout could have dissuaded Peyton for taking any action; instead, his words serve to draw Peyton into the act of sabotage that results in his being caught and hanged. Again, why?
This was a time of war. Peyton was not a soldier, but as a potential saboteur, he was the enemy. By giving him information, the scout was determining whether Peyton was a danger or not. When it became clear that Peyton was a threat, the scout effectively sets him up for failure. No doubt the Northern soldiers were prepared, waiting for Peyton's attempt to burn the bridge. When he was caught, he was indeed hanged, with both military ceremony and precision. His punishment would then serve as an example to other civilians who might consider trying to commit acts of sabotage against the invading Northern forces.