WASHINGTON — The Obama administration’s decision to authorize the killing by the Central Intelligence Agency of a terrorism suspect who is an American citizen has set off a debate over the legal and political limits of drone missile strikes, a mainstay of the campaign against terrorism. The notion that the government can, in effect, execute one of its own citizens far from a combat zone, with no judicial process and based on secret intelligence, makes some legal authorities deeply uneasy. To eavesdrop on the terrorism suspect who was added to the target list, the American-born radical cleric Anwar al-Awlaki, who...