What point is Bierce making with the sentinels' indifference to the action at the center of the bridge and with the statement that they "might have...

It is not a flattering description of these soldiers.  At the center of the bridge, a man is about to die for his beliefs, for trying to stand up for his principles.  Neither we (I should hope) nor they would agree with what he believes -- Peyton Farquhar is described as a slave-owner and secessionist who would like to see the South leave the Union in order to preserve its way of life -- however, the man is, nonetheless, about to lose his life.  Even though the Union soldiers, cannot abide by the choices Farquhar has made, especially the choice to try to burn down the Owl Creek Bridge in order to disable the railroads and prevent efficient communication for the Union army, they must on some level agree with him that "all is fair in [...] war" at least or else they would not appear to condone the execution of a civilian without a trial, without allowing him to say goodbye to his wife and children.  Their complete lack of emotion, their lack of empathy for a fellow human being in the moments just before he is hanged, is an indictment of the behavior of both sides in a war.  It isn't just the Peyton Farquhars who are to blame for the war; there are men behaving badly, without reference to their humanity, on both sides.