I think once the source is dead, the duty to protect the source becomes a lot less important. But it seems Assange now holds key evidence to what must be an on-going murder trial. He really should turn that over to the police and make an appropriately scrubbed version available to the public. If he cares about justice for this sources, this is what he will do. If his source really was murdered for leaking, and the perpetrators get away with it because Assange withheld key evidence, that adds to the danger of all the future whistleblowers. But if Assange contributes to catching the murderers, that should deter would-be murderers of future whistleblowerrs.