If Manning were aware of particular abuses, why didn't he release documents only about those particular abuses? If he wasn't, why did he release anything? This sounds like a case of "I know the evidence is in there somewhere, so let's throw it all out there and see what sticks." When prosecutors do this, we call it a "fishing expedition", and we don't like it at all. Why should we like it _more_ when a vigilante does it?
If I am pissed at my employer and decide to steal and publish all the internal documents I can, and I get lucky and some of them are incriminating, that doesn't excuse what I did. It just means that both I and my employer are criminals, instead of only me.