1) The US was doing a lot of things wrong. Going off the 2011 cables [0] they were spying on various people they weren't meant to be, there were one or two things that look war crimes to me but who knows technically and a few gems like "Der Spiegel reported that one of the cables showed that the US had placed pressure on Germany not to pursue the 13 suspected CIA agents involved in the 2003 abduction of Khalid El-Masri, a German citizen".

2) It wasn't obvious in that leak that the US was doing anything counter the interests of the US. But Assange isn't a US citizen and wasn't in the US at the time, so that isn't a reasonable standard to hold him to.

3) Even internally to the US though there is a reasonable argument that he was helpful. If US citizens don't have easy access to this sort of information, how are they supposed to effectively exercise democratic control on the government? People are going out and doing terrible things in their name which, arguably, are counterproductive and they would probably not want done. Accountability requires sunlight and they can't debate whether there is enough sunlight without people like Assange.

4) It turns out that the US does have a huge probably-illegal certainly-ill-advised spying program that was being sniffed out by leakers. The response to Assange seems likely to be part of a campaign to keep material information on such topics like that out of the public sphere.

[0] https://en.wikipedia.org/wiki/United_States_diplomatic_cable...