Earlier quoted context omitted.
Wrecklessly and without simple redaction, in such a way that very well might have resulted in the deaths of US officials. And highly questionable selective publishing. I think the guy is getting fucked over in this unfairly, but let's not pretend he has released information to a reasonable standard.
All of this is over the Manning leaks, where Assange tried to go over the documents with the US government so he could do a responsible disclosure. The US refused. He then got a team of journalists together to sift through the documents to make sure things were properly disclosed. Some Guardian journalists then published the encryption keys in a book.
Snowden's mistake was revealing himself. He should've done what the Panama Papers leaker did.