The Marines ain't going to take care of this guy. He is an ex Marine, which means they have no more jurisdictiction over him. But I bet he will be dealt with.
I too am concerned about the loyalties of some of our naturalized citizens. If we went to war with their former governments how would they stand? Is this question ever asked of them, No, I am not painting all naturalized citizens as traitors. We have a lot of of homegrown traitors in this country to, the Walkers and Hansen come to mind.
"I, _____, do solemnly swear (or affirm) that I will support and defend the Constitution of the United States against all enemies, foreign and domestic; that I will bear true faith and allegiance to the same; and that I will obey the orders of the President of the United States and the orders of the officers appointed over me, according to regulations and the Uniform Code of Military Justice. So help me God." (Title 10, US Code; Act of 5 May 1960 replacing the wording first adopted in 1789, with amendment effective 5 October 1962).