Did he actually take up arms against the United States? Do we have evidence of that? If so then there would be clear justification, but even then an open and public record should have been made of it in court. Otherwise there should have been a trial and conviction. Don't you think its a little dangerous for the executive branch to declare a U.S. citizen an enemy and kill them without trial?
We need to avoid executive declarations and people being judged by secret star chambers.