He was disillusioned by all that defined Iraq. When immediate withdrawal didn't occur in 2009 and the killing continued it was the final straw.

He held out long enough and then it appeared as though the end was not happening.
Bradley Manning is as much a victim of the illegal and immoral war into Iraq as the nation has been. We were powerless, too. The difference was he wasn't. He had some degree of power to change the circumstances of many people including Americans. 
He is not treasonous. He attempted to stop what no politician had the backbone for. Iraq was already held to public trial. The majority of Americans had already decided. The war was wrong. He could not believe we all were caught up in a meat grinder without end.
By Richard A. Serrano

July 26, 2013, 8:57 a.m.

...Coombs displayed (click here) in large blue-and-white letters copies of Manning’s personal emails, some after his 2009 deployment to Iraq as a classified intelligence analyst, others around his arrest in early 2010 for the largest breach of U.S. secrets in the nation’s history.

“I’m more concerned about making sure that everyone — soldiers, Marines, contractors, even the local nationals get home to their families,” Manning emailed.

“I place value on people first,” he said in a later email. “One of the bad parts of the job, having to think about bad stuff,” he emailed further.

In yet another email Manning described to a fellow Internet chatter his angst over whether to start disclosing classified data. “What would you do?” he asked, if you knew explosive material was stored away in a “dark room in Washington” that if exposed would reveal the inside story of the Iraq and Afghanistan wars, the inner workings of the State Department, and how terror detainees are treated atGuantanamo Bay, Cuba.

“That is a whistle-blower. That is somebody who wants to inform the American public,” Coombs told the judge, Army Col. Denise Lind.

Coombs played for her a video clip of a now-notorious U.S. Apache helicopter firing indiscriminately at a group of civilians, killing and wounding journalists, children and others.

“Look at this from the eyes of a young man who cares about young life,” Coombs told the judge....