Bin Laden Killing: the Legal Basis - Council on Foreign Relations: "The killing is not prohibited by the longstanding assassination prohibition in Executive Order 12333 because the action was a military action in the ongoing U.S. armed conflict with al-Qaeda and it is not prohibited to kill specific leaders of an opposing force. The assassination prohibition also does not apply to killings in self-defense. The executive branch will also argue that the action was permissible under international law both as a permissible use of force in the U.S. armed conflict with al-Qaeda and as a legitimate action in self-defense, given that bin Laden was clearly planning additional attacks.
Some critics of the administration's legal theory that the United States is in an armed conflict with al-Qaeda might--if they were consistent with their past criticisms--argue that the United States did not have a right to use military force against bin Laden outside of Afghanistan, and that Washington should instead have sent an extradition request to Pakistan or asked the Pakistani government to arrest bin Laden. But such traditional critics may prefer to remain silent in this instance."