Jack Goldsmith is a Harvard Law professor and a member of the Hoover Task Force on National Security and Law. He served in the Bush administration as assistant attorney general in charge of the Office of Legal Counsel. His new book is Power and Constraint: The Accountable Presidency after 9/11.

He recently wrote at Foreign Policy, Fire When Ready (12 Mar 2012), which overall is a good summary of the points covered in this thread re: targeted killings (emphasis added).

His BLUF:

While the Obama administration can improve its public explanations for targeted killing, its critics have wildly overstated the legal concerns about the practice. Even exaggerated criticisms, however, can serve a useful role. As I detail in my book, an important lesson of the first decade of indefinite war against al Qaeda and its affiliates is that relentless and sometimes brutal scrutiny and criticism of the presidency from all quarters forces the presidency to engage in self-reflection and public justification that, in the end, strengthen it. The criticisms of targeted killing have produced public debate and limited judicial scrutiny of targeted killings that have enhanced the legitimacy of the practice. They have also encouraged the executive branch to tread very carefully and to provide much more public information and explanation about its operations than usual. There is room for improvement, of course, but we should not be blind to how deeply the Obama administration's targeting killing practices are embedded in the rule of law.
Agreed, except I would make it clear that the primary justification has to be based on the Laws of War. Those can, of course, be included within the rule of law as broadly defined (as LawVol, for example, has pointed out).

If one views policy as being the primary driver for whether targeted killings are or not employed, then Jack's citation of the polls is a critical element:

These disclosures have fostered a robust public debate about targeting killing in the United States and abroad, and the American public broadly approves of what it sees. According to a recent Washington Post/ABC News poll, 83 percent of respondents (including 77 percent of liberal Democrats) say they approve of the Obama administration's use of drones against terror suspects overseas, while only 11 percent disapprove. The approval/disapproval numbers drop to 65/26 percent when respondents are told that the targets are American citizens. As the Washington Post's Greg Sargent noted, "65 percent is still a very big number." Sargent added that "Democrats approve of the drone strikes on American citizens by 58-33, and even liberals approve of them, 55-35."
Given this political support, the political issue of targeted killings appears closed among USAians. Of course, lawyers and non-lawyers, academics and non-academics are free to argue the issue one way or the other. Thus, the comments to Jack's piece are interesting.

An interesting comparison is that roughly the same numbers oppose US air strikes and US ground force intervention in Syria (more oppose ground force intervention). And, roughly the same number support withdrawal from Astan by 2014 or sooner.

Regards

Mike