It's more complicated than most posters are making it out to be.
Assange's organization released classified documents which exposed American war atrocities under the Bush (and Obama) administrations. For instance, this was leaked:
https://web.archive.org/web/2016043.../wiki/Camp_Delta_Standard_Operating_Procedure
Along with other documents about the torture of prisoners of war (a war crime).
In addition, Wikileaks exposed that the Obama administration was lying about hostile casualty totals, or camouflaging civilian murders as being elimination of enemy hostiles in order to obfuscate the actuality of the conflict to the American people.
So Assange and Wikileaks did great good, at first; there's a lot of stuff that they leaked and I'm just providing two quick examples. Ultimately though Assange has become a controversial and reviled figure. He has been
accused of rape (it is incorrect to say that he is a rapist as of yet) by the Swedish government, and charged with a count of rape as well as three charges of molestation. The lesser charges have been dropped, but there is still a Swedish warrant out for his arrest for the rape charge.
Julian Assange is an abrasive person and I agree with the general consensus here that he's an asshole. The Ecuadorian embassy gave him asylum and he took advantage of their generosity in countless ways. There is also credible evidence that he acted as a mouthpiece for Vladimir Putin and his Russian regime to harm America's reputation in the world. Though I'd argue to that point that George W. Bush and Barack Obama's actions in the Iraq and Afghan theaters and their handling of the "war on terror" in general did far more to harm the world's opinion of the United States than anything that Assange or Russia did.
There's a lot of information out there about Assange and Wikileaks. It's worth taking a few hours to read up on it. The situation around Assange is complex.