I would agree that this is one of the themes of The Killer Angels. However, as with all books there are multiple themes and ideas expressed. It's especially true in a novel such as this where the narrative is presented from many points of view.
The Killer Angels is a character study of several of the principal officers who took part in the Battle of Gettysburg. The main focus on the Union side is Joshua Lawrence Chamberlain; on the Confederate, James Longstreet.
In the Longstreet chapters Shaara does not so much tell us the general's motivation for taking part in the war as analyze Longstreet's intellectual prowess and independence of mind. Much is made of Longstreet's dissenting from Lee on the way the battle should be fought. Longstreet's defenders (of whom Shaara is one) believe that Longstreet's proposed tactic of swinging the army to the south and placing it between the Union forces and the city of Washington would have won the battle for the Confederates. Most historians, however, counter that this plan would probably not have been workable.
The Chamberlain chapters do focus more on the underlying reasons for the war, and Chamberlain's endorsement of them and wish that others, both field officers and soldiers, would be inspired by the same factors. But through all the chapters, what Shaara seems to stress more than anything is the intense effort and heroism men on both sides put into the battle, regardless of how strongly they "believed in" the principles over which the war was fought. It's obvious that Longstreet was not an enthusiastic, fire-eating secessionist. Like his fellow Confederate Armistead and like W.S. Hancock on the Union side, he longs for the days before the war when the country was at peace, and there was solidarity among career officers regardless of what section of the country they came from. This yearning for camaraderie among men is, perhaps, the overriding theme of The Killer Angels.