Killing U.S. Citizens with Extreme Prejudice Seems to Exceed the President's Authority

SEE HERE Where do you draw the line when it comes to due process. St. Paul said "I am a Roman citizen" and brought the proceedings against him to a dead stop. He had to be sent to Rome for a trial. It seems that under certain circumstances we're comfortable with a president ordering the execution of an American citizen because he's decided he's a despicable human being. What if he decides that you're a despicable human being? Where are the checks and balances? This is something well worth thinking about.