It's vigilantism, pure and simple.
I already have a system for deciding when and how to release government information in the national interest. It's called Government. It's not a perfect system, but it does have the advantage of including elected and appointed officials, judicial review, and the consent of the governed.
Maybe Assange thinks he has a better system, but I don't recall electing him. I don't recall any of my elected representatives appointing him. I don't believe he's answerable to the intelligence oversight committee, or the chairman of the joint chiefs of staff of the armed forces, or any other organization I have established and empowered to oversee national security and state secrets.
That said, if he believes that it's worth becoming an outlaw--with all the consequences of that decision--to reveal this information, then I wholeheartedly encourage him to step up and do the right thing. Reveal the information, and accept the consequences. And I sincerely hope he's right.
But I don't assume he's right just because he's donned the sacred ZOMGWHISTLEBLOWER mantle. And I don't see why my hand shouldn't be against him, if he insists on unilaterally crowning himself World Czar of State Secrets.