Excellent post, AD. But it really isn't unexplainable- first you go through a process of deindividualizing the enemy captives- humiliate them, take away their rights and make them all look and seem the same- it dulls them to what you do to them, decreasing resistance. Also, you dehumanize them- label them infidels, terrorists or something of that sort.
Add in the fact that the troops fight with become like family- the fact that you lose so many becomes frustrating enough to push you to the breaking point- the above two psychological techniques make it very easy at that point for a good person to do terrible things. It however should never remove accountability. At some point, without even knowing it, you risk losing everything you have been fighting for and becoming everything that you thought you were fighting against....such a fine line.
Quite a shame that although there have always been more villains than heroes, it seems more hopeless than ever from that standpoint