Here is an interesting analysis of opinions about the legality of Death of Osama bin Laden. I guess lots of experts disagree on this issue. (from Wikipedia)
"University of Texas School of Law Professor Robert M. Chesney said that it was lawful to kill bin Laden "if he's doing anything other than surrendering".[192] Martin Scheinin, the United Nations Special Rapporteur for Human Rights, said: "The United States offered bin Laden the possibility to surrender, but he refused. Bin Laden would have avoided destruction if he had raised a white flag."[199] Professor Matthew Waxman at Columbia Law School, an expert in national security law, said "under international law, U.S. forces would have substantial discretion to use lethal force given that this was a military operation against an enemy commander likely to pose a very serious threat to U.S. forces".[193]
Benjamine B. Ferencz, one of the former chief prosecutors at the Nuremberg trials,[202] questioned the legality of killing and said it would have been "better to capture bin Laden and send him to court ... Killing a captive who poses no immediate threat is a crime under military law as well as all other law." He also claimed that "the issue [with Bin Laden's death] is whether what was done was an act of legitimate self-defence".[203] Australian-born British human rights lawyer Geoffrey Robertson said that the killing risked undermining the rule of law. "The security council could have set up an ad hoc tribunal in The Hague, with international judges (including Muslim jurists), to provide a fair trial and a reasoned verdict."[204] British law professor Philippe Sands QC, speaking to the BBC, acknowledged that under what is known as the doctrine of necessity, where there is an "overriding threat to national security", such an act might not give rise to responsibility or liability, but warned that that argument was made more difficult against a background of a rise in extrajudicial killings, including through the use of drones".
Louise Doswald-Beck, a former legal chief for the Red Cross, said that bin Laden was clearly not an enemy combatant. "He was basically head of a terrorist criminal network, which means that you're not really looking at the law of armed conflict but at lethal action against a dangerous criminal."[205] Nick Grief, an international lawyer at Kent University, said the attack had the appearance of an "extrajudicial killing without due process of the law."[194] Human Rights Watch said "law enforcement" principles should have applied.[198]"
