Why did Carter have to shoot the man on the horse?

After having fallen asleep while on duty, Carter wakens and looks across the valley to spot what looks like a statue of a soldier astride a horse on the cliff above. Shaking aside his drowsy phantasms, he realizes that it is not a statute, but an actual Confederate on a horse. Realizing that this can only mean be a scout who is, like himself, familiar with the terrain, he has only once choice if he is to save his fellow Union soldiers from being spotted and slaughtered. So he raises his rife, sets his sight on the target and fires.