He was a person.
And in my government class, we were taught that the use of the word "person" in the Constitution was deliberate. The fifth amendment refers to "persons". Other portions refer to "citizens". I don't understand the contrast between the killing of al-Awlaki and the killing of bin Laden and it's disconcerting to me that people think rights like due process only matter for citizens. That should be disconcerting for you too.
Due process is of course dependent on circumstance. Due process on the battlefield is different from due process regarding prisoners of war. Due process for a criminal pointing a gun at a cop is different from due process for a criminal in hand-cuffs. Who the hell cares about "citizenship". Due process is a right that attaches to persons.
I don't personally know the ins and outs of combatants vs. soldiers, etc. But I do know one thing - I don't see this killing of al-Awlaki as any different from the killing of bin Laden. But I'm bothered by the fact that (1.) some people seem to think that non-citizens somehow ought to be treated differently when it comes to the rights of persons, and that seems dangerous, (2.) people are talking about totalitarianism with regards to this, which to me trivializes totalitarianism, and (3.) that people still talk as if radical Islamic terrorism is a criminal issue is really baffling to me.