What about the right of life to those he was plotting to kill? At what point do their rights begin to matter?
This fucker left this country with no plans to return (unless he was to return with explosives in his shorts). He declared war on these United States and we have evidence that he was actively involved in past acts of war/terrorism (not going to get into semantics) against us and was plotting future acts against us as well.
This is was not a criminal that we could send the cops out to arrest and then try before a jury of his peers. This fucker is was an enemy of the country operating from foreign soil. Period dot.
By the way, there is plenty of precedent for this. For example, there were American citizens of German descent in WW2 that went to fight for Germany. We didn't try to arrest those guys, did we? Fuck no. They were enemy combatants regardless of citizenship.
American lives were just saved and no one should miss a wink of sleep over that.