Seriously, I think people defending the soldiers here aren't listening.
The question is NOT about their ACTIONS, but their ATTITUDE during said actions. As pointed out by Tekeo, there is a part where the firing soldier is practically asking for the wounded man to go for a gun so he can finish him off, there are multiple "hoorahs" going on during the firing, and you can practically HEAR the "high fives" going around.
well.... You are train to be a killing machine. I kinda like the line that "Gross pointe blank" pointed out. First you do it, then it become a job, then you might actually like it. Part of the psychological conditioning I guess. Is it wrong? I don't know. People have to be in that state of mind in order to pull the trigger and kill another human being. The convention of war does not allow to shoot an unarmed person (at least that is what I believe)
Now, the soldier could have pet up aggression against the enemy. This group of enemies (or the people as a whole) may already have killed his brother in arms in past excursion and want to exact revenge within the rules of war. I don't know. I'm guessing. Of course there are some people who just revel in the destruction of another human being (as long they are the enemy). In War, you need people who are willing to follow order and pull the trigger against another person. some people can mentally adjust and "switch on/off" but some can't. This is why there are psychological help after they come home to "turn off" that "killing mode"
Edit: now I personally don't like war. It is a messy product and just bad for both sides. Even if you win, you still lose a lot of good people, civilians and not to mention property damage. The job dictates you to do certain things, and human beings have ways to "reason" the "rights and wrong" just to get by. I have a friend who I call my brother served 3 tours as Army Ranger. He use to be a real fun guy, always mess around and stuff, but after he came back (now a game tester) you can sense sometime has change in him. He still try to live a normal life, but he has change.