It's called being human. I imagine, when you're stuck in a foreign country, knowing or watching people you know die, that you're going to get scared. And you'll probably have to get that fear out of you. So, when you've got a bunch of people who are the 'enemy' in a prison, what's wrong with taking it out on them? They're the enemy, after all, and it makes you feel better. Like a drug.
The purpose of the army is to kill other people's armies. We don't have fancy-pants force fields; if someone attacks anyone, it's kill-or-be-killed.
But murder is one of the worst crimes that society thinks a person can commit. Someone who respects other people and is open-minded makes a great member of society, but a poor soldier. Someone who cares far more about themself than they do other people at large makes a great soldier, but a poor member of society. Finding a balance between those two is hard.