People are killed without due process all the time. Criminals, crimes of passion(eye roll), murders, acts of defense, accidents, acts of terror and such). I know Anwar al-Awlaki was an American citizen but in his instance and with people like him, I feel that being an open and admitted enemy of the US revokes his rights as an American and he -should- be treated as a terrorist and get no rights at all. I would gladly accept that exception to the legal process and I still feel it was the right and just thing to do, to kill him.
Now before some people here go apeshit, let me state that it should only be for people/citizens, who have admitted, by acts, words and deeds, they are enemies of the US and opposed to it by any means. Those people should, I believe, have their citizenship revokes and be treated like the terrorists they are.
Is that hard? Yes, but with the enemies we're fighting, this needs to be looked at and we cannot afford to give them full rights. Look at how long the trial lasted for the sheik that bombed the World Trade Towers in the early '90's.