If Manning had released only the video of the helicopter atrocity, then he is a whistleblower. He saw something horrific that was not being dealt with by the chain of command, and he acted. If that was his only "crime", I'd be on his side.
But it wasn't his only crime. He proceded over a period of weeks to download more than 700,000 classified documents and dump them onto the internet without having the slightest clue as to what was contained in those documents. They could have contained spy lists, confidential informant lists, classified military procedures and reports... he didn't know and he didn't care.
That's treason. And that's why I'm not on his side.