Trump would never have been elected if it wasn’t for Assange. He exposed Clinton. He also knows who gave him the emails.
Assange was quoted as saying something to the effect that everything negative about Trump was already public, but lots and lots of negative stuff about Hillary and Podesta were trying to stay hidden.
He hates secrets. He operates out in the open, in that way he and Trump are similar. But Julian is a libtardarian while Trump is a conservative populist.