FQ, being clueless you may want to review your "military law".
Following orders you know are illegal is a criminal offense, whacking prisoners is considered a war crime, and assassination was specifically outlawed during the 70's.
Wrong Tom. Legally its a lot more complicated, and a hell of a lot more entertaining. Here's a very brief legal overview. It starts with Ford and ends with BO, with segues though Carter, Reagan, W. and BO. You know it has to be good.

Here's the deal. Ford signed Executive Order 11905 prohibiting assassinations for political purposes after the Church Committee Report. That was strengthened (by Carter

) in Executive Order 12036. It basically kept the spooks out of dealing with US citizens, leaving that exclusively to the FBI, but also just flat out banned assassinations. Reagan signed Executive Order 12333 stating the same ban. However all three are Executive Orders, not law, and can be countermanded, quite lawfully, by simple Presidential order under authority of the Comander in Chief clause in Article II. A Presidential Finding that someone poses a clear and present danger, with notification of the "Big Four" has been held to be the gold standard here, but it is not legally necessary. An Executive Order for actions taken abroad can be counter-mandated by another, and the military is obligated to obey as it is Constitutionally lawful.
As exhibit A, I give you W.. In Executive Order 13470 signed in 2004, he basically reorganized the Intelligence Community into the shape we see now, in order to make it fall into line with The Patriot Act (among other concerns). However, hidden in and amongst the the delegation of responsibilities for who was responsible for what, was this little gem:
4) Conduct covert action activities approved by the President. No agency except the Central Intelligence Agency (or the Armed Forces of the United States in time of war declared by the Congress or during any period covered by a report from the President to the Congress consistent with the War Powers Resolution, Public Law 93‑148) may conduct any covert action activity unless the President determines that another agency is more likely to achieve a particular objective; Essentially, this says that any covert activity, of what ever nature, provided it has Presidential and Congressional approval (in the broadest sense) is kosher. This means the "clear and present danger" test is gone, as is the need to notify the "Gang of Four". Congress, by giving vague approval to actions in Iraq and Afghanistan, as well as the War on Terror, have essentially given the green light for a President to use this clause to order a hit. Who used it? Not just W., but also BO. W. green lighted Bin Laden, though BO was in charge when Bin Laden died. However, it was an all BO operation when we took down radical cleric, terrorist, and US citizen Al Awlaki in Yemen. It was the CIA/The Air Force that did the deed with a drone, but the the deed was done. All perfectly legal Tom, because Congress approved action against Al Queda. Actually, it was even Kosher under International Law, (as though we care) , because the UN named Al Awlaki as a member of Al Queda and International Law recognizes the right of nation states to use force against declared enemies.
This was a long post, but I hope a useful one. The moral of the story is this. With the exception of political assassinations which we seem to have sworn off post Castro, the US does engage in assassination and it is legal as long as it has Presidential sanction and takes place abroad. Even US citizens abroad can be considered legitimate targets.
FQ13