I'm not sure that there is any due process issue here. He was killed on the battlefield. I believe there was several instances of this happening in WW2 in Europe. Yes, the american citizens weren't directly targeted in WW2, but there wasn't really much in the way of directly targeting anyone in WW2. In the new world where we target individuals in war, it was only a matter of time before an american national was found on the battlefield to be fighting against his or her own country. He should have expected to have the same amount of attention as any other AQ operative. However, if he was given more attention or captured and executed without trial, we would have a different conversation on our hands. But again, this my personal opinion.