Remember: against all enemies, either foreign or domestic!
I agree this person is firmly on the enemy side, and could easily be found guilty of treason, but we also must remember the Fifth Amendment "No person shall be held to answer for any capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, ... nor be deprived of life, liberty, or property, without due process of law;"
And the US Supreme Court has extended these rights to non US Citizens, if we state they have renounce their citizenship by actions, with Obama being quoted after the ruling as saying ""a rejection of the Bush administration's attempt to create a legal black hole at Guantanamo" and "an important step toward re-establishing our credibility as a nation committed to the rule of law." This administration is not creating a legal black hole, it's just plain ignoring any rule of law.
If he was killed on the field of battle, that is justified. But saying if you find him and it's not convenient to capture him kill him, that?s overstepping the line. The only thing that is known for certain that this person has done is speak out against the actions of the American Government, his first Amendment rights, he is suspected of more but "believe that he has become a recruiter for the terrorist network, feeding prospects into plots aimed at the United States" (from linked article) is not enough.
If speaking out against America or what our government has done is enough to get your citizenship revoked maybe we should start with the Clinton?s and Obama?s.