'Osama bin Laden was not in our custody. He was an active soldier in the war that he himself declared against the United States. In war one does not have to give enemy soldiers the opportunity to surrender. For example, bin Laden did not give the occupants of the World Trade Center that opportunity (in contravention to the law of war bin Laden drew no distinction between military and civilian targets). Under the law of war enemy soldiers may surrender if they choose. But they had better be quick, clear, and explicit that they are surrendering. Bin Laden could have surrendered to us at any time over the past decade, but he chose not to. When he heard the U.S. helicopters overhead he could have rushed out of the compound with his hands in the air and thereby protected his wife and children, but he chose not to. Nor did he raise his hands when our soldiers encountered him. It was his choice, and there is no doubt that it was lawful for us to kill him.
Other al Qaeda leaders such as the Egyptian Ayman al Zawahiri and the traitor Anwar al-Aulaki may profit from bin Laden's example and decide to surrender and stand trial or they may decide to continue to wage war against the United States. It is their choice.'
http://www.ohioverticals.com/blogs/akron_law_cafe/2011/05/debate-over-lawfulness-of-bin-ladens-killing/