This video from the USA says about itself:
Obama Administration: US Forces Can Assassinate Americans Believed to Be Involved in Terrorist Activity
Anwar al-Awlaki was selected for assassination as a test case that, if successful, would set a precedent according to which the executive branch of the US government has the unreviewable power to secretly liquidate its political opponents, including US citizens: here.
Yale Law Professor Bruce Ackerman: “The killing of an American citizen without due process is a national scandal”: here.
Execution by secret White House committee: here.