Secondly, he was engaged in a war against the United States. Imagine a German American in 1942 going to Germany and joining the Wehrmacht and shooting at Americans. Do we worry he was killed without "due process?" There is a regular disease in our society where everyone worries about the criminal and not the victim.
Kill them, kill them all.
Obviously true. Even more so when they are illegal enemy combatants and operating in territory outside of US jurisdiction. By extension of the authors argument, all Al Qaeda would have to do is keep one American with each unit and we could never attack them.