It's easy to lose compassion when you decide the you are at war and that your enemy will stop at nothing. It's easy to justify anything that will give you any advantage no matter how small and to minimize any negative effects. Easy to say "Yeah, I made children soldiers and sex slaves, but I gave them a better life than my foe would have. I am at war. I need to do whatever I need to do to keep my troops in line."

It happens so often in ways a million times more subtle than this.