It's pretty offensive to muslims to lead your characterization of al-Awlaki with the fact that he was Muslim, rather than the fact that he went to Yemen and took up arms against the United States while inciting violence against the country. U.S. Citizenship isn't a blanket immunity against U.S. military action. It's certain that the U.S. killed Americans who went and fought for the Nazis in Europe. Few people would challenge the validity of such actions. Does it become wrong because you make a list instead of doing it indiscriminately?
And ultimately, it's deeply counter-productive to try and lump people like Hammond together with people like al-Awlaki. Military action against foreign aggressors is a fundamental right of sovereign nations. People will not give it up. If you tell them that such actions must be subject to "law" all you'll accomplish is distorting and diluting the law to accommodate the sorts of actions that must be taken against foreign aggressors. The law should not try to insert itself into foreign military matters. It's not a fight it will win, and it will be the downfall of the law.