The healthy human mind doesn't wake up every morning thinking that today might be it's last day alive either. War is not healthy. Pissing on them was probably their therapy and their revenge. Wrong, yes, but understand why.
They weren't 'getting off' on it. Yes, they will probably need therapy and will have PTSD, but in an insane environment, people do insane things. It is war. The fight for them is NOT over until they are out of the country, and even then many still have to fight PTSD, or fight with rehabilitation after losing limbs. Sometimes their fight is NEVER over.