Was the al-Awlaki Killing Legal?
Was it? I don't know -- the question hinges on facts I don't claim to possess. (I know -- that kind of agnosticism seems to infuriate people for some reason).
Here's a good run-down of some of the arguments, courtesy of Spencer Ackerman.
It is, of course, an open-and-shut case once one chooses a legal framework to apply. And therein lies the rub -- should the act be judged according to the laws of war, or international law (which is itself a complex mish-mash of treaties, norms and customs), or some other framework.
It's worth keeping in mind the hierarchy of laws, according to the Constitution. The Constitution itself is the supreme law of the land. But treaties, once ratified by Congress, trump simple legislation. That means that the Authorization to Use Military Force (AUMF) against terror groups -- cited to justify much of the extraordinary measures undertaken in the "war on terror" -- is subservient to our commitments under the UN Charter, the Geneva Accords, etc.
Anyway, whatever one' view, a good rule of thumb is to ignore anyone who doesn't recognize the rather unambiguous fact that domestic and international law are different creatures.