I find it very strange that people talk about soldiers as if they had some special moral responsibility for what happens during a war.
Sure, they should be thinking about the implications and ethics of their actions, as we all should, and yet they also signed up to serve the nation, and to do what their nation thought necessary and just. Do people think that democratic nations should not have armies, and it is an ethical imperative for Americans, English and others to refuse to serve? If not, then it is all of our responsibilities, as voters and citizens, to ensure that only just wars are fought, and using ethical means. If not, then we are all responsible, not just the soldiers.
Instead of asking "how could you kill someone" we should be asking "how can you live in a political system that causes people to be ordered to kill people, in which you have the ability to change this fact".
I'm not a pacifist myself, I just don't see how in a democracy, the buck stops with the soldiers.