He released completely true but negative information about Hillary Clinton. That seems to be enough for some people.

He seems to have published no embarrassing or damaging information on Russia (at least in latter years?), and seemed willing to serve their purposes when it came to the 2016 election, by releasing hacked Democratic Party emails timed to do maximum damage to Hillary Clinton's campaign.

Prior to that, I had considered Wikileaks a brave experiment in radical transparency. Since then, I've considered Wikileaks a somewhat biased source. The truth is the truth, yes, but every truth is partial, and context matters.

"Asset" is perhaps too strong a word, but "useful idiot" may apply, or "the enemy of my enemy".