According to the principles of self-defense enshrined in the UN Charter, the nation-state has a right to protect itself when attacked. Notwithstanding important questions regarding the limits and legality of pre-emptive self-defense, Bin Laden's continued threats and his proven ability to successfully conduct attacks—9/11 in particular—unequivocally categorized him as a legitimate target at the time he was killed. The attack, therefore, was not an act of retribution under international law. It also adhered to fundamental international law principles, including distinction, military necessity, proportionality and alternatives. As a result, the operation was the manifestation of lawful and legitimate self-defense.
Amos N. Guiora is Professor of Law at S.J. Quinney College of Law, the University of Utah;
http://www.jurist.org/forum/2011/05/amos-guiora-targeting-bin-laden.php