This seems like speculation. You can easily say that they're too tired to do anything. I think where armies are undisciplined (no punishment), or respect for the non-combatant or combatant is non-existent many will do what they want.
The worst rapists in a modern army were probably the Russians after the fall of Germany. I'm sure the Germans did some nasty things. The Japanese were pretty horrible.
Many will not becuase many have a moral outlook on life. Some becuase they don't like to hurt people. Almost everybody likes to think of themselves, whether religious or not, as good people.
Originally Posted by Druid_Elf
True, but there is also a different reason behind it. Many soldiers who are on the front lines have high stress levels due to war. After a few days soldiers on the front line survive by following the commands that have been hammered in during their combat training. How your actions will be after you have been true days of near-dead are different than a normal person who knows that the end of the world is near. I doubt that stress levels will be the same, because knowing the world will end is more liberating for the mind I believe (since you know you can't avoid the unavoidable).