An elementary moral arithmetic would seem to indicate that those who suffer great sacrifices and run great risks to do evil are worse characters than those who do evil easily or offhandedly, even though the former might be more aesthetically satisfying: they are willing to buy their evil at a higher price. However, if one wants to get rid of the evil instead of the evildoer, one must turn one's attention to _it_ and note that war crimes are made possible only by war and war is made possible by the State and such popular doctrines as Just War and Humanitarian Imperialism. (The capitals serve to warn of oxymora.) We are back at the issue of the effectiveness of punishment.
But -- if one must do evil in the form of war -- the wise combatant will treat all prisoners of war as well as possible, since they are often useful sources of information, and can be traded to the enemy for one's own, or for other benefits, or kept as hostages. (One of the American government's complaints during the War in Vietnam was that prisoners of war were being kept near desirable targets, but given they were carpet-bombing the whole country, it is difficult to know what the Vietnamese could have done with them.)