Did he not have good reason to be angry at his government?
During his duties he came across crimes being committed by Iraqi Federal police. From his chat with Adrian Lamo, the two-faced rat who turned him in:
Manning: was watching 15 detainees taken by the Iraqi Federal Police… for printing “anti-Iraqi literature”… the iraqi federal police wouldn’t cooperate with US forces, so i was instructed to investigate the matter, find out who the “bad guys” were, and how significant this was for the FPs… it turned out, they had printed a scholarly critique against PM Maliki… i had an interpreter read it for me… and when i found out that it was a benign political critique titled “Where did the money go?” and following the corruption trail within the PM’s cabinet… i immediately took that information and *ran* to the officer to explain what was going on… he didn’t want to hear any of it… he told me to shut up and explain how we could assist the FPs in finding *MORE* detainees…http://www.wired.com/threatlevel/2010/06/wikileaks-chat/So he comes to find out that dissidents in Iraq are being arrested and disappeared into Iraqi prisons for the "crime" of being concerned about corruption in Maliki's government... and his superior officer effectively tells him "So what? Just find us more detainees!" If the account is true, he was being used by his government for suppressing democracy in Iraq! If you were in his shoes, wouldn't you feel compelled to do something about it?
Maybe releasing all those documents was the only way Manning could think of to get back at this corrupt government with its bullshit imperialist war efforts abroad. Yet, instead of holding him up as a hero, this government chooses to detain him indefinitely and persecute him for his brave act. Some democracy we live in.