The dilemma arises from the ambiguity of the term "extrajudicial killing." Such killings are quite common (and often quite justified) in the United States in the form of "officer involved shootings." The crucial distinction is whether the killling is retributive (i.e. as a consequence for past actions) or pre-emptive (necessary to prevent future harm). It is easy to sympathize with Mr. Williams's unease with retributive extrajudicial killing, while also puzzling why he apparently does not recognize the legitimacy of pre-emptive killing.
Pre-emptive killing (just like pre-emptive war) is understandably disquieting; it seems a fundamental principle that punishment never be applied prospectively, either for things that people might do but can't or can do but won't. Simple fairness requires a rebuttable presumption that a person will not commit an act that would justify pre-emptive violence. The reality of life is however that such presumption is overcome by circumstances. Had a Jefferson County deputy been a better shot and dispatched Eric Harris in the midst of the Columbine rampage, the act would have been justified as preventing future shooting victims, but not as retribution for those already dead. The subtlety however is that the prior killings created circumstances by which to conclude that Harris poses a future threat, and therefore cold be killed on the spot.
A policeman who confronts a recidivist felon, who then points a gun at the officer is preumably justified in using lethal force to defend himself. It is understood that the fatal shot that the policeman fires is pre-emptive, i.e. to prevent something that circumstances indicate that the felon is at a high risk of doing, but which he has not done yet; kill or injure the policeman. this killing is "extrajudicial" in the sense that there has been no prior judicial determination that the felon poses a threat to the policeman, such a determination lies totally in assessment of the immediate circumstances. If the object of "extrajudical" violence does not pose a prospective threat, then the means of justification disappear. If, for example Osama bin Laden was bed-bound, non-communicative and paralyzed after a stroke, partitioning his cranium with a nine millimeter bullet, while perhaps emotionally satisfying, would make for an uncomfortable precedent in a civilized democracy.
The issue then, is not whehter the authority to kill someone is judicial or political, or whether the target is an American citizen or not, or in this contry or another. The question is whether a person's prior conduct and present circumstances present a sufficient threat of future harm to justify killing to prevent that harm. Necessity requires that we vest police officers with some discretion to make such a determination in emergencies, and our system of government vests certain officials with making analogous determinations with respect to people such as bin Laden and al Awlaki. As always such discretiion may be abused, and we then rely on other institutions to remedy such abuses.
Mr. Williamson is justified in wondering whether the death of al Awlaki is a justified pre-emptive killing, or an extrajudicial retributive killing, or perhaps suspicious that it is the latter masquerading as the former. He may legitimately doubt that al Awlaki posed a sufficient threat of future terror to justify elininating him without due process. He is right to be uncomfortable at the prospects for abuse, but he shold not use this discomfort to deny that occasionally pre-emptive killing is justified by the protection of innocent life.