"The issue here is whether what was done was an act of legitimate self-defence," said Benjamin Ferencz, an international law specialist who served as a prosecutor during the Nuremburg trials and argued that 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 told the BBC World Service.
Like Mr Ferencz, British law professor Philippe Sands QC says it is impossible to make a definitive legal judgement without knowing precisely what happened. But he says the case for the raid's legality has been weakened.
The raid to kill Bin Laden took place at a villa in the town of Abbottabad
"The question to ask is: were the measures taken in the actual situation that pertained reasonable and proportionate, given the circumstances in which the [Navy Seals] found themselves?" he told the BBC.
"The facts for Bin Laden don't appear to easily meet that standard.
"As a matter of international law, one country is not free to enter another country apparently without the authorisation of that country, and intervene, whether to kidnap or kill a national of a third state," Mr Sands said.
He 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 he said the difficulty with that argument was that it comes against a background of a rise in extrajudicial killings, including through the use of drones, and that this was not a "lawful direction to be taking".
The logical conclusion of any idea that Bin Laden could be killed as an enemy combatant was "that anyone associated with al-Qaeda in any country in the world can be taken out, can be executed," Mr Sands said.
"I think it's deeply troubling if we are indeed moving to a place where you can have a global assassination policy for those who are perceived to cause trouble," he added.
"In certain exceptional cases, use of deadly force may be permissible as a measure of last resort... including in operations against terrorists," they said in statement.
"However, the norm should be that terrorists be dealt with as criminals, through legal processes of arrest, trial and judicially decided punishment," they added.
"Actions taken by states in combating terrorism, especially in high profile cases, set precedents for the way in which the right to life will be treated in future instances."
Source BBC News   Aidan Lewis

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