Check the wikipedia article, pretty neutral and factual reporting on the history. TLDR he revealed the US committing war crimes, they went after him with everything they had including planning an assassination attempt (which they never went through with). They tried to apply US law internationally to somebody who wasn't a US citizen and wasn't in the US. The UN said his detainment was illegal and torture. He's been on the run, in some embassy, or jail for over 10 years for activity other news organizations regularly and legally engage in (leaking classified documents). Various US military, intelligence, etc agency heads have testified to congress that they couldn't find a single death related to the documents he leaked, he didn't put anybody at risk, in fact, Wikileaks sent every leak to the US govt before leaking it asking them for notes on what to redact. The US refused to participate in that process.

He also revealed the DNC was trying to bury Bernie, which the DNC didn't even deny, they had to let a bunch of their top people go and do a bunch of primary reforms as a result. That's when liberals started hating Wikileaks, because the DNC emails helped get Trump elected. They say the "timing" of right before the election makes his leak partisan. But wouldn't you want that information before you vote? It is the job of wikileaks, or any journalist, to maximize the impact of information they are revealing on corruption. It's not Julian's fault the DNC was corrupt AF, all they had to do to avoid that was... not be corrupt.

There were also some sex assault allegations against him, which I tend to believe have some veracity to them however the accusers explicitly did not want him charged, it was a ploy to get him to Sweden where he would be extradited to the US. He was never even charged, only "wanted to questioning" but somehow got an interpol notice for it. His lawyers offered over a dozen times for him to be interviewed but Sweden insisted on an "in-person" interview for some reason. Curious.

Oh, and he helped save Snowden's life by getting him a flight out of China.

Published documents from the Iraq and Afghanistan war that outed Iraqis and Afghanis who were helping the US.

Received dirt on the American Democrat party from the Russians and released it during the election to help Trump.

Raped a woman and hid in the Ecuador embassy until the statute of limitations ran out.

Ran Wikileaks. Received classified US military docs about wars in Iraq and Afghanistan. Published them.