Manning's statements about it seem to suggest that (s)he views it as an excuse for what (s)he did.
This is contrary to every statement of his that I've read/seen summarized. IIRC he said at one point that he should have been removed for psych reasons. Now I don't remember reading whether that was for "I'm trans" reasons, or "I'm disillusioned with the military" reasons, or even "I tried to report war crimes to my superiors but they just shut me up" reasons (though I suspect it was one of the latter). But even if it were for being transsexual, the fact they failed to remove him has never been presented as an "excuse" for what he did. What he did
needs no excuse: he revealed evidence of numerous specific violations of international law committed by the United States. Having discovered such evidence, he was
obligated to release that information internationally.
As for why he released
so much information instead of just select documents on the specific and indisputable
war crimes (such as the "Collateral Murder" incident)...
I think he was aware there would be retaliation against him, and that even if he won in court with a whistle-blower defense he would still have been denied any further access to those databases. Databases which, were he to continue perusing, he could reasonably expect to uncover a multitude of similar instances of bad **** the United States was up to. He couldn't just give the documents to WikiLeaks one at a time as he discovered them: he would be found out and silenced after one or two times. Better to take the whole database and give it to a publisher he can trust to disseminate it publicly
and discreetly (see below).
I believe WikiLeaks generally looks through the information they are given and for the most part avoids publishing things that would be
directly useful to Al Qaeda, e.g.: cryptographic keys, deployment schedules, etc.. There was one case where they leaked the names of undercover agents (possibly via Manning?), but I believe that is the exception, not the rule.