Juan Cole describes exactly why the assassination of Awlaki was illegal and immoral, and provides an alternative that could have been utilized:
The two possibilities are that al-`Awlaqi was an enemy combatant on the battlefield in a war on the US, in which case obviously the US government has a right of self-defense and can kill him with impunity; or that he is a civilian terrorist, in which case the US constitution would give him certain prerogatives, such as trial by jury before execution.
It is, however, difficult to see in what way al-`Awlaqi could be configured as a soldier in an enemy army with which the US is actively at war. He was an American citizen of Yemeni extraction (and dual citizenship), living in Yemen. That he was an American is not very relevant to this issue. You can be an American and still be an enemy combatant, as with the German saboteur born in the US, who was sentenced to death as an enemy combatant after WWII for sabotage in the early 1940s. But the United States is not at war with Yemen, and al-`Awlaqi is not in the Yemeni military anyway. The idea that, legally speaking, the US could be at war with small terrorist organizations such as al-Qaeda strikes me as a non-starter. A rhetorical flourish such as the “war on terror” is not a legal statute or article in the constitution. The killing of al-`Awlaqi differs from that of Usamah Bin Laden because in the latter case a US expeditionary force was confronted with someone who appeared to be going for a weapon, whereas al-`Awlaqi was simply targeted…
If al-`Awlaqi was a civilian, could he have legally been killed in this way? It has been pointed out (by Newt Gingrich and Salman Rushdie) that he was a traitor and a terrorist.
Such a position hearkens back to the idea of the “outlaw” in common law. A person declared an outlaw by the king was deprived of all rights and legal protections, and anyone could do anything to him that they wished, with no repercussions. (The slang use of “outlaw” to mean simply “habitual criminal” is an echo of this ancient practice, which was abolished in the UK and the US). There is a similar idea in Islamic law, of the mahdur al-dam, someone whose blood can be shed at will. Muslim legal authorities can give a fatwa or legal ruling that an individual falls into this category because he committed an offense such as blasphemy, in which case any Muslim may kill him with impunity. Ironically, this is the category into which Salman Rushdie himself was put by Ayatollah Khomeini in 1988. Likewise, the Baha’i religious minority in Iran is often considered by conservative Shiite clerics to be mahdur al-damor outlaws, resulting in their persecution. The same forces in US society so worried about sharia being enacted in the United States seem actually to want to adopt the medieval Islamic legal notion of the outlaw in this case, and apply it to an American citizen abroad.
The problem with declaring al-`Awlaqi an “outlaw” by virtue of being a traitor or a terrorist is that this whole idea was abolished by the US constitution. Its framers insisted that you couldn’t just hang someone out to dry by decree. Rather, a person who was alleged to have committed a crime such as treason or terrorism had to be captured, brought to court, tried, and sentenced in accordance with a specific statute, and then punished by the state. If someone is arrested, they have the right to demand to be produced in court before a judge, a right known as habeas corpus (“bringing the body,” i.e. bringing the physical person in front of a judge).
I really recommend reading this in it’s entirety. I’m not going to paste the whole thing because of it’s length, but Prof. Cole does a great job of ensuring that your time isn’t wasted and the entire piece is extraordinarily informative.