I can accept the idea that Manning's actions were crass, irresponsible, stupid, and cowardly. Instead of seeing some injustice and leaking information of that injustice to the outside world, he chose to just grab everything he could and dump it. In retrospect, the information that he leaked was probably not dangerous to anyone, and it did, indeed, expose deep tentacles of corruption in the US government. However, there is no way that he read everything that he leaked, and he did, as you say, just send massive amounts of classified information -- most of which he had no idea of the content (because there was too much to read) -- to a foreign third party with a sometimes unclear agenda.
HOWEVER, none of this warrants torture, and as an American I hope that Manning's lawyers win their trial. It is an unprecedented chilling effect and incomprehensibly unjust that, in the United States of America, a foolish whistleblower would be tortured to set an example for future whistleblowers. Torture of any kind, mental or physical, is clearly unconstitutional and is unquestionably both anti-American (as in, it betrays the values that we base our country's existence on) and evil.