The argument is completely immaterial. If, for example, there is an active shooter in a shopping center, school, or neighborhood, he is a clear and present danger to others, and the focus is on stopping him. "Due process" happens for the most part after the perpetrator is in handcuffs. If he won't stop shooting and put down the gun, the SWAT team is not denying him "due process" by stopping him.
Surely we would have captured al-Awlaki alive if we could have, and a fit of legal hand-wringing about every step of how his case was handled would ensue, just as lawyers were standing by in case bin Laden were taken alive.
Another option would have been to try him in absentia, but there would then have been complaints that it was a show trial, and he could not defend himself. And he would not have been "able" to defend himself because he could not be captured in order to be present at trial. The argument begins to go in circles. Meanwhile, al-Awlaki was actively waging war against the people of the United States, and was an ongoing and obvious danger.
This operation was not a petty, grudge-based "assassination" by an annoyed government "whacking" a citizen who turned against it. But this commentary notably avoids discussion of the threat al-Awlaki posed (calling him a "marginal figure"), and the plots he was behind or incited in order to support that impression.
"The due-process-free assassination of U.S. citizens is now reality," by Glenn Greenwald for Salon, September 30 (thanks to Mackie):
It was first reported in January of last year that the Obama administration had compiled a hit list of American citizens whom the President had ordered assassinated without any due process, and one of those Americans was Anwar al-Awlaki. No effort was made to indict him for any crimes (despite a report last October that the Obama administration was "considering" indicting him). Despite substantial doubt among Yemen experts about whether he even has any operational role in Al Qaeda, no evidence (as opposed to unverified government accusations) was presented of his guilt. When Awlaki's father sought a court order barring Obama from killing his son, the DOJ argued, among other things, that such decisions were "state secrets" and thus beyond the scrutiny of the courts. He was simply ordered killed by the President: his judge, jury and executioner. When Awlaki's inclusion on President Obama's hit list was confirmed, The New York Times noted that "it is extremely rare, if not unprecedented, for an American to be approved for targeted killing."
After several unsuccessful efforts to assassinate its own citizen, the U.S. succeeded today (and it was the U.S.). It almost certainly was able to find and kill Awlaki with the help of its long-time close friend President Saleh, who took a little time off from murdering his own citizens to help the U.S. murder its. The U.S. thus transformed someone who was, at best, a marginal figure into a martyr, and again showed its true face to the world. The government and media search for The Next bin Laden has undoubtedly already commenced.
What's most striking about this is not that the U.S. Government has seized and exercised exactly the power the Fifth Amendment was designed to bar ("No person shall be deprived of life without due process of law"), and did so in a way that almost certainly violates core First Amendment protections (questions that will now never be decided in a court of law). What's most amazing is that its citizens will not merely refrain from objecting, but will stand and cheer the U.S. Government's new power to assassinate their fellow citizens, far from any battlefield, literally without a shred of due process from the U.S. Government. Many will celebrate the strong, decisive, Tough President's ability to eradicate the life of Anwar al-Awlaki -- including many who just so righteously condemned those Republican audience members as so terribly barbaric and crass for cheering Governor Perry's execution of scores of serial murderers and rapists -- criminals who were at least given a trial and appeals and the other trappings of due process before being killed.
From an authoritarian perspective, that's the genius of America's political culture. It not only finds way to obliterate the most basic individual liberties designed to safeguard citizens from consummate abuses of power (such as extinguishing the lives of citizens without due process). It actually gets its citizens to stand up and clap and even celebrate the destruction of those safeguards.