Your question is disingenuous because it ignores the factual context. Due process is inherently concerned with the particular factual circumstances. It has never meant that you get a full trial in every circumstance. So no, the U.S. government is not entitled to kill a U.S. citizen once he leaves U.S. soil as a categorical rule. But it can do so if it provides process commensurate with the circumstances. Which brings us back to the question: What process is "due" to someone who commits something that is not a crime, but rather is an archetypal act of war, from foreign soil?