The Trump admin are going after him for leaking false or misleading statements to the press as an anonymous source, which is highly likely to have happened as it's something basically all FBI Directors do but usually everyone just allows it.

The bigger reason is his involvement with the Clinton private email server investigation, which is at least somewhat dodgy. Clinton almost definitely broke the law, because even Comey's final investigation report specifically said that it amounted to "gross negligence" which the Justice Department then stepped in and asked him / made him change to something like "extremely careless" because gross negligence is an actual crime and they didn't want to have to prosecute a potential president with a crime or announce charges during an election.

In more general ghoul activities that I'm aware of I know he was a pretty hardline anti-privacy, anti-encryption guy. He very publicly went after Apple for not giving the FBI and law enforcement an easy backdoor into their products and data. Even the NSA/CIA director at the time at the time came out and said the request went too far and was attempting to end privacy to make 'his job easier'. Admittedly, this is probably because they could do it anyway, but they didn't want to share with the FBI.

Whilst the CIA/NSA were certainly the main culprits (and suggested the most extreme action like kidnapping and assassination) of the Wikileaks prosecution/criminalisation, Comey's FBI were enthusiastic participants too. I believe he tried to argue for designating various Wikileaks members and even related journalists like Laura Poitras not as journalists but as "information brokers" in order to investigate/prosecute them not as journalists but as potential foreign assets.

Pretty much anything the FBI got up to between 2013-2017 was on Comey's watch, including breaking it's own rules and (already extremely liberal and invasive) guidance on spying on activist networks like Occupy Wall Street who were still going, the Keystone XL pipeline protestors, environmental campaigners, and of course people associated with BLM.

He's always been a ghoul though. He was a rare FBI director I was aware of as a Brit before he got the position, as from 2008 onwards the HSBC bank was in a constant state of scandals here (financial crisis, bailout fraud, fixing Libor rates, tax avoidance & dodgy slushfunds) and they brought Comey in to be their top lawyer during perhaps the most obviously egregious one which was when they were caught actively facilitating global drug cartels (probably at least some CIA linked) and international organised crime more generally. I remember reading about that and discovering that his previous gig had also been at Lockheed Martin.

Comey approved waterboarding