Here's a scenario under which Walker's guilt is less than death-worthy. Does it fit the known facts? Seems to as far as I've heard. If you've heard otherwise, please chime in.
Remember that the Taliban have only been our "enemy" since about October, and only after they refused to hand over bin Laden as Bush knew they would do (since they were religiously obligated to do so), and thus Bush knew he could paint them as an enemy by making this demand without giving any proof to back his claims. Before October (or very late September), the Taliban were not in the picture other than as yet-another-odious regime in some far-off land. A regime I have been condemning for years (yes, I have heard of them long before 9/11), but not one that wsa an enemy of the U.S.
Furthermore, Islam didn't have a bad name in the U.S. until post-9/11.
So now we have young, idealistic Walker who did what so many of us did: he joined a religion different than that of his parents. Ah, rebellious, idealistic youth! Anyway, he's studying Islam from his religious teachers, and grooving to what he was learning. He wants to take it to next step and fight for his beliefs (sounds just like some rabid JW's, doesn't it?), but his religious teachers tell him he's better off as a student, not a fighter.
Being young, ignorant, and idealistic, he wants to fight for his new cause anyway. So he disappears from his religious teachers and family and heads to Pakistan. Meanwhile, his parents frantically try to find out what has happened to him (nowaytess, here is your answer: I did know they didn't have contact with their son. DId you know they very much wanted contact, but that HE cut it off?). His religious teachers try to figure out what's happened to him. But he disappears.
Now he's getting Taliban training in Pakistan. Remember, 9/11 hasn't happened yet, and they are not yet our enemy. He is in a far-off land, doing his religious zealout thing as so many of us did, and it has nothing to do with America. After his training, he goes to Afghanistan to fight the Taliban cause, again, having nothing to do with America at this stage.
Now 9/11 happens. Does new travel well in Afghanistan? Nope. And what you hear is filtered by the Taliban. Do you think Walker got the straight story? Of course he didn't. He got propaganda. He probably hadn't a clue about 9/11 the way it really happened, but believed whatever lies the Taliban told him.
Still, the bombing begins and he freaks out. After all, this is his own country attacking, and now what does he do? Leave the country? He hears that any Taliban caught at the border are arrested. Can't do that. He probably sticks to his friends and tries to dodge bombs.
Then he is captured. When the news of his capture first hit, there was some confusion about him and who he was and what he was doing when he was caught. The stories of his fighting came out later on. Who knows what he was actually doing? Maybe he was just dodging bombs and trying to stay alive, still unsure what the Americans were doing to his new friends, and still clueless to the big picture of terrorism.
So he's captured, and interviewed, and still unsure of all the events going on he gives a witness about his faith, just like we would have done once upon a time, all us ex-idealistic Witnesses when confronted with the facts for the first time. Now he arrives in America, a prisoner, and he hears the entire story. Think he's having second thoughts about his choice now? Just like we did when we learned the truth about the WTS?
Did he fight against America? Or was he just in the wrong place at the wrong time? None of us know yet. Perhaps at the trial we will learn. Until then, calling for his death may be premature. Time will tell.