We now have a law review answer.
1. The Administration sought to conduct the operation according to the international laws of war, which allow the killing of enemy operational commanders. (My addition: though to satisfy a public thirsty for bloody revenge, it played down its actual un-American respect for civilised global standards.)
2. The legal basis rested on the finding that Bin Laden was a currently active operational leader. A simple reprisal killing would not have been lawful.
3. The Seals were instructed to take Bin Laden alive if he surrendered and the surrender “could be safely accepted”. (My addition: The “safely” part is a bit contentious. Soldiers are required to take significant, but not suicidal, risks in combat to avoid harming innocent civilians; logically this should extend to enemies taking themselves out of the combatant category.)
The surrender option was of course very unlikely. Max Hastings gives the chance of success for an individual German soldier trying to surrender on the Western front in 1944-45 as about evens. It was higher for organised bodies of troops, for instance in a surrounded town. It must have been less for operations carried out indoors and at night, as this was. Night vision goggles weren’t available in 1945, but I don’t suppose they allow you to judge body language very well, and expressions at all. I’m not saying this is right, but it’s hard morally to condemn soldiers in high-speed combat for playing safe and treating practically any animated gesture or movement by identified hostiles as threatening.
Bin Laden had thirty years, since his name came to the attention of Soviet Spetsnaz around 1980, to consider the real possibility of such an attack. He did have the option, once he knew his compound was under attack, to await capture motionless with his hands above his head. If he did this, and the Seals shot him anyway, they committed a war crime. Otherwise, and we have no reason to challenge the Administration account, they and it are in the clear.