Ever since the World Wars, we have had rules about engagement in conflict that supposedly stop things like the slaughter of enemy combatants in non-hostile situations (such as surrendered or retreating foes), the claiming of spoils of war, and the use of violence on non-combatant civilian populations.
And D&D, in its most famous form, is a group of mercenaries entering an enemy camp, slaughtering every man, woman and child, and claiming their treasure. In the real world, any group who did this would be in the Hague. (Well, should be. Without getting into RW politics, the enforcement of war crime laws is not exactly stellar). Then again, there is probably a reason why WotC has updated two dozen different classic modules to 5e but KotB isn't among them.