What does that have to do with releasing everything? Even Snowden didn't publish everything he took (although he presumably gave it all to the Russians)
Maybe he is a true believer in "radical transparency", and thinks governments shouldn't have secrets.
Or he was "acting out" because of his psychological issues.
He didn't release everything.
Wikileaks did. (I don't remember if Wikileaks actually released EVERYTHING but either way it sounds like they leaked TOO MUCH.)
Regarding his motives- this is a man who volunteered to die for his country.
Here's some things he wrote in private messages BEFORE he got caught.
"hypothetical question: if you had free reign [sic] over classified networks for long periods of time ... say, 8–9 months ... and you saw incredible things, awful things ... things that belonged in the public domain, and not on some server stored in a dark room in Washington DC ... what would you do? ...say ... a database of half a million events during the iraq war ... from 2004 to 2009 ... with reports, date time groups, lat-lon locations, casualty figures ...? or 260,000 state department cables from embassies and consulates all over the world, explaining how the first world exploits the third, in detail, from an internal perspective? ..."
Though yes, he did commit a crime, he was also mentally unwell and I can imagine that could have affected his poor decision making. "Back at Fort Drum, (s)he continued to display emotional problems and, by August 2009, had been referred to an Army mental-health counselor."