Just to have the key facts on the table, as most of you know, Anwar al-Awlaki, unlike virtually all the other members of al Qaeda targeted by the US, was an American citizen. He was born in New Mexico of Yemeni parents, returned to Yemen when he was seven and then returned to the United States for college. More recently he returned to Yemen and became involved with the al Qaeda affiliate al Qaeda in the Arabian Peninsula.
So how is it exactly that a US President can order the extra-judicial killing of an American citizen? I agree. It's a troubling thought. But I would approach it differently. Is it really possible that a US citizen can be operating abroad as a key leader of an international terrorist group, organizing and inspiring terrorist attacks within the US, and the US has no recourse other than to secure an arrest warrant and arrest him somewhere in the wilds of Yemen? That strikes me as preposterous.
The key in my mind is that al-Awlaki is or was essentially waging war against the United States from abroad.
Now, there's a great deal we don't know and probably won't know. And that leaves open a great opportunity for abuse. There is for instance some controversy over whether al-Awlaki was simply inspiring terrorist attacks or whether he was operationally involved in them. I don't think a US president should ever be allowed to do this within the United States or anywhere else where arrest is a credible option. I also think that in the case of targeting a US citizen, the President of the United States should need to personally authorize the decision. Someone has to take responsibility and be accountable.
The idea that his US citizenship mandates that he be handled through the US criminal justice system in every case, even the most extreme ones such as this, simply doesn't make sense to me. So I just can't credit that argument that targeting him is 'illegal'.
Just to emphasize the point again, this is a case where there's a ton we don't know. So I'm answering the question based on my understanding of what happened. Liable to abuse? Sure. Illegal. I can't see the argument, either legally or logically.