Bin Laden is not a special case at all. And it has nothing to with how 'evil' he was.
There are basically two ways to deal with terrorists and insurgents--that is to say, with people who wage war, but not under the authority of a recognized sovereign state. You can deal with them as criminals, who are subject to criminal law. Or you can treat them as soldiers, who are subject to the laws of war.
Each approach has its advantages and disadvantages. If you criminalize terrorists and insurgents, for example, you deny them legitimacy. If you treat terrorism and insurgency as a military problem, by contrast, you loosen the rules of engagement considerably, as I described above.
I used to believe that terrorists and insurgents should be treated as criminals, but lately I've swung around to the opposite view. I think they should be taken at their word, and treated as soldiers waging war. And insurgencies should be treated as what they are: a contest for political power, and for the monopoly of authorized violence.
The advantages of this approach are numerous. You can use much greater force against insurgents and terrorists if you treat them as an enemy in war. You aren't obligated to try to capture them. You can detain them indefinitely, without charge, as prisoners of war, until their "government" surrenders. Since they generally violate the laws of war anyway, you can still prosecute them for their war crimes if you want, when it's all over. And since they see themselves as soldiers anyway, they can hardly complain about being treated like soldiers.
On the government side, military forces are under military law and discipline, and less likely to commit atrocities than police and paramilitary forces. And discipline is absolutely crucial in this kind of asymmetric warfare.
There are only two ways to defeat terrorists and insurgents: by wholesale massacre and counter-terror, the way the French Revolutionaries crushed the Vendee; and by slow attrition, the way the British wore down the IRA in Northern Ireland. Wholesale massacre and terror are obviously not acceptable. That leaves attrition--which requires iron discipline in the face of insurgent and terrorist provocation.
In addition, by treating captured insurgents and terrorists as POWs, instead of treating them like criminals, you can also make the conflict less atrocious by not provoking reprisals from the enemy in response to the mistreatment of their prisoners.
It also eliminates arguments like this, about whether or not terrorists should be captured or killed. If they surrender, they should be captured. If they don't, they should be killed. If they want to wage war, then let it be war--on both sides.
supererogatory--morally praiseworthy, perhaps, but hardly morally obligatory.
Ah, so
that was the word young Spock was defining!
Cool word, eh?