In Part Two, the Federal scout, who is disguised as a Confederate soldier, stops at Peyton Fahrquhar's estate and requests a drink. During his visit, the Federal scout informs Peyton that the Union troops are rapidly advancing and have reached the Owl Creek Bridge, where they have constructed a stockade on the northern bank and issued an order declaring that any civilian caught interfering with railroad or bridge will be hanged. The Federal scout then suggests that a civilian could easily overcome the single sentinel guarding the bridge and halt the Union's advance by setting fire to the bridge. The Federal scout's visit is significant because he baits Peyton into attempting to destroy the bridge, which results in his hanging. After speaking with the Union spy, Peyton undertakes the mission himself, which results in his hopeless predicament on top of the Owl Creek Bridge and directly leads to his death.

The reason that this is important is that this guy who stops by for a drink is going to entrap Farquhar.  That is pretty much his whole purpose -- to go around and entrap people who might be inclined to help the Confederacy against the Union.

When this guy stops by for a drink, he also lets Farquhar know about the bridge and how important it is and how there might be a good way to burn it down.  When he does this, he puts the idea in Farquhar's mind and this leads Farquhar to go and try to destroy the bridge.

So the fact that this guy is truly a Union soldier leads to Farquhar's death.