"If an American citizen went off to join Hitler's army during World War II, would there have been any question that this alone would make it legal to kill him? Why then is there an uproar about killing an American citizen who has joined terrorist organizations that are at war against the United States today?"
Congress officially declared war against Nazi Germany. Have we done so against each terrorist organization? Further, during WWII I don't believe it was legal to specifically target a specific individual for death on the say so of the president. Did it happen? Maybe. Was it legal? I doubt it then and I doubt it is now.