The efforts to discredit Assange are a primary example of a successful smear campaign. It's not surprising that people are gullible to make such campaigns easy, character assassination always works when large government agencies are behind it for many years. I was actually surprised how long it took.
Here's the thing, though, Assange and WikiLeaks are complicated figures who have done good and evil things. They are fair targets for criticism, and it's genuinely ok for someone to draw the conclusion that, "given the criticism, I cannot support them". Yes, a deliberate smear campaign exists, but also these are institutions with complex histories. You cannot simply call anyone critical of them "gullible".
What evil things, specifically?