Another principle of law, however, is that criminals should not benefit from their own wrongdoing -- in this case, his fleeing the U.S.
One can argue rationally that Anwar al-Awlaki was (1) an uncapturable fugitive from justice and (2) an imminent threat to the United States. Both characteristics were of his own volition.
Under those exact circumstances, what does due process look like? The answer cannot be that the U.S. has no course of action but to wait indefinitely. Yemen had already informed the U.S. that they would not extradite al-Awlaki even if they had the motivation and the ability to arrest him. In reality, Yemen had neither -- although they did assent to a waiver of national sovereignty for the assassination.
There was, in fact, a process within the Obama administration to place al-Awlaki on a target list. Given the publicity of more than 12 months, al-Awlaki knew or reasonably should have known that he was on the list. During this time he could have surrendered to U.S. authorities.
Does this pass my personal sniff test for due process? Almost. I would have preferred to see al-Awlaki undergo trial in absentia, either in the U.S. or pursuant to a Nuremberg/Tokyo-like process before being placed on an assassination list. Absent such a process in place, I would have preferred that President Obama explain to the people why he was unable to wait for such a process to be implemented.
That said, I will not join those who decry the assassination absolutely. I strenuously oppose capital punishment in criminal cases, but I am not a pacifist and I have no problem with judicious use of deadly force in military situations.
I have friends on both the left and the right who will disagree with me, although not necessarily for the same reasons.
