Jeremy Waldron in the LRB blog:
The killing of Osama bin Laden is an instance of a much more general policy pursued by the United States and its allies – the targeted killing of named individuals in the war against terrorism and against various insurgencies in Iraq and Afghanistan. In the midst of American celebration of the fact that al-Qaida has lost its charismatic leader, it is worth getting clear about targeted killing in general, i.e. about the legality and the desirability of a policy of this kind. Targeted killings are of two kinds. The first involves killing people who are actually engaged in carrying out terrorist acts – planting a bomb or preparing someone for a suicide bombing. The second involves the elimination of high-profile individuals whose names appear on a special list of active commanders and participants in terrorism or insurgency. These killings are part of a strategy of disruption and decapitation directed against terrorist organisations.
Killings of either kind can be problematic from a legal point of view (the more so if they involve the violation of another country’s sovereignty). But they may be lawful depending on the circumstances. What is certainly forbidden is targeted killing for the sake of justice or vengeance, both of which have been cited by American authorities as justifications in this case. Calling the killing of Osama bin Laden ‘justice’ summons up very primitive notions of retribution, utterly dislocated from the rule-of-law processes that the civilised world prides itself on. If justice in the strict sense had really been in question, an attempt would have to have been made to capture the man alive. Despite some initial equivocation, it is plain that no such attempt was contemplated.
I think the only possible justification for the shooting of bin Laden was as a legitimate act of war.