In assessing the legality of the killing of Osama bin Laden one is reminded of a saying about the situation in Lebanon. If you think you understand it, it has not been properly explained to you.
Of course, one major obstacle is that we do not have all the facts. However, we also do not have all the law.
The complexity of analyzing the legality of the killing begins with the threshold issue of applicable law. Is the conduct to be analyzed according to domestic law or international law? If domestic law, then which country’s domestic laws are applicable? Certainly that of the United States and Pakistan would be applicable. Saudi law might also apply (e.g., on the basis of nationality), in addition to the laws of those countries that have another basis under their domestic law for exercising extraterritorial jurisdiction (e.g., on the universality principle).