I think one reason for the "Assange is an asshole" hate is that there are many people all over the West who love and believe in the USA as a unequalled force for good, and they are filled with resentment towards Assange for destroying that belief.

They can't deny the terrible acts perpetrated by the US, so they try to reclaim some moral superiority by accepting and repeating the completely unsubstantiated (and in some cases, ridiculous) smears against Assange.