The guy was not playing dead he was hurt. The marine calmly raised his gun and shot. He didn't take cover or even act like he was worried, the man on the floor was clearly unarmed.

As for the soldiers being in a shitty situation, maybe they are, but they chose to be soldiers. They were not conscripted, they knew that they could be sent to war. Soldiers should not enjoy war, that indicates some serious problems.