I think in alot of cases the superiors are to blame, I saw an interview with a Sergeant who said he'd been told by his commanding officer to run down civilians if they got in the way of the truck, men women or children. He said he took it with a pinch of salt but he knew younger guys who were slowing down purposely as if to let kids pass, then speeding up and killing them.
I think if you put young minds (17-21) that are essentially a blank canvas in these situations into the hands of experienced leaders who have a way of justifying every terrible action thinkable. 90% of these guys are gonna simply do as thery're told, and conform with one another. The last thing an 18 year old kid would wanna be (especially in a far off country) is an outcast.
If you put so many young guys in this situation & tell them what they're doing is ok, eventually it's going to become Grand Theft Auto, where they completely detach themselves from the lives they're taking & the wrong they're doing, and start having fun.