This happens everywhere and all the time. It's not particular to any race, religion or war.
Just like the Police, some people become soldiers because they are petty minded little classroom bullies who are too cowardly to do it without some kind of 'higher authority'. Yes, the 'Few bad apples' line sounds old hat, it has been used a lot of times, because in most cases it is true.
I think it is disgusting the way these prisoners have been treated, and I hope the soldiers themselves are publicly stipped of duties, made to apologise and sent home in disgrace, while the locals watch, that is important.
It's just people being people, don't for one moment think any conquering army has behaved any better, and a great many have behaved worse.