To prevent the major crime of torturing and murdering innocents, he committed the minor crime of leaking the evidence. He has spent the last seven months in solitary confinement – a punishment that causes many prisoners to go mad, and which the US National Commission on Prisons called "torturous". He is expected to be sentenced to 80 years in jail at least. The people who allowed torture have faced no punishment at all. Manning's decision was no "tantrum" – it was one of the most admirable stands for justice and freedom of 2010.
A traitor? Maybe, but then how many people throughout history now looked on as heroes were traitors to someone?