Er, they could have joined the Peace Corp, Red Cross, etc. They didn't accidentally wake up one day and kill someone, they made a series of choices and put themselves into a series of events where they got to use powerful weapons against people.
Think about how bad the friends and relatives of the innocent people they killed feel.
-- Good point. I'd add that in addition to the US soldiers who made a choice to serve their country, and who must take responsibility for those actions, I'd add that the terrorists/insurgents made similar choices. They chose to dick around with AK-47's in a war zone. They made a choice to allow kids to be around them, knowing they would become targets.
Similarly, the cameramen made a choice to hang around insurgents who were dicking around with AK-47's. And the people in the van chose to drive up into the middle of a battle, without red cross markings, to pick up wounded terrorists who were dicking around with AK-47's and RPGs.
So yes my friend, we all make choices. Sometimes they turn out to be dumb ones, or will at least seem like dumb ones when someone like you is watching them on video, and doesn't understand their context, 4 years after they are made.
Cheers.