There’s very strong evidence that this dude committed treason against America. Yes, he did not get a trial. Perhaps he should have been tried in absentia. Then upon conviction he would have been a legitimate target, based on the points you are bringing up.
I have to refresh my memory about John Walker Lindh, the dude who was fighting for the Taliban. Why exactly is he in prison? He’s an American citizen.
I think our laws need updating to deal with cases in which American citizens are the enemy.
We shouldn’t be afraid to try people for treason in scenarios like this. Lindh was convicted on a host of weapons and conspiracy and other charges.