... when a group of combatants are badly outnumbered, or surrounded, or otherwise very, very unlikely to win a conflict, they have a considerable incentive to surrender--but only if they believe they will subsequently be treated with mercy. That is why individuals, and nations, surrender. If, by contrast, a group of combatants believes that, by surrendering, they are only making themselves vulnerable to further harm--specifically torture and/or death--they have no incentive at all to stop fighting.
The humane treatment of surrendered captives, therefore, is a crucial--arguably the crucial--understanding between adversaries if their conflict is to end in any way other than with the wholesale slaughter of the losers. It's worth noting, too, that it is not merely the lives of the losers that are preserved. If they do not surrender, it may be that they are all killed; but it is very likely that, in the process, they will also kill some, perhaps even many, of the eventual victors.
... it's my understanding that many armed conflicts that we might consider pre-civilized concluded with just this kind of slaughter (and pillage, enslavement, etc.), and that the widespread recognition of civilized rules of war has saved literally countless lives. As bad as the Nazis were, I think it's unequivocally a good thing that we were not forced to depopulate Germany. The reason we weren't was that Germany surrendered, and the reason Germany surrendered was its well-placed faith that we wouldn't depopulate (or torture, enslave, etc.) the nation anyway ...
http://blogs.tnr.com/tnr/blogs/the_plank/archive/2009/04/28/the-difference-between-battle-and-torture.aspx