The legal justification basically comes down to the fact that the incredibly broad 911 Authorization of Use of Military Force (AUMF) would justify lethal force.
This appears to me to be good law, but remain dubious of the facts.
Basically, and this is on a quick reading of a heavily redacted memo full of legalese, there is no mention of the actual activities that al-Awalki engaged in that had him declared a combatant, just a justification for lethal actions against American citizens who have assumed a combat role against the United States.
So, we still don’t know what he did to be declared a combatant. It could be that he was involved in major military decisions, functioning as a military officer in al-Qaida, but I’ve never seen any sort of release, either officially or through leaks, claiming this.
What we do some of what he was doing.
He produced and distributed sermons supporting Jihad, and we know that he provided religious advice to people in AQAP, including the Underoos bomber, Umar Farouk Abdulmutallab.
If these are the facts that led the US government, and I am inclined to believe that this is the totality of the actual facts against him. (See my no leaks comment above)
If this is the case, then the US state security apparatus specifically targeted Anwar al-Awlaki on the basis of activities which are purely clerical in nature.
This begs the obvious question, “When do we start droning the leaders of Operation Rescue?”
After all, if pastoral support of terrorism rates assassination, the ongoing terrorism against abortion providers should be at the top of the list.