yes it WAS legal

Yglesias » Killing Osama Bin Laden Is Legal: "In a domestic context, I think the 9/11 AUMF has been put to some questionable uses since its passage but if authorized anything surely it authorized military action against Bin Laden. Meanwhile, Bin Laden considered himself to be at war with the United States. In an international law context, the use of military force is plainly authorized by UN Security Council Resolution 1368 which uses the magic words “inherent right of individual or collective self-defence,” “threat to international peace and security,” and “all necessary steps.” That’s how the UN Charter says “bombs away.” And this has all been reaffirmed since Bin Laden’s death by the Security Council and the Secretary General of the United Nations."