Well as far as killing for the wrong reasons because your forced to. The reality is you always have a choice. I remember a World War 2 story. It was the winter of 1942 outside Stalingrad. A Russian soldier was caught and taken prisoner. A German officer ordered an infantry soldier to shoot him. He refused, and after further direct orders to shoot him he still refused. The German officer then pointed a pistol in his face and ordered him once more, he refused knowing very well he could most likely be shot for his disobedience. The officer then stomped away in anger. Sadly as the unarmed Russian tried to escape someone else shot him. Point is you always have a choice, even if that choice leads to your death. Which stresses that if someone can't accept the fact they could die in military service they shouldn't be in the army.
As for taking lives. The way I see it, you shoot them or else they shoot you. If you have prisoners, you treat them like any other human being.