1) Democrats and Republicans both wanted his head over horrendous moves his department made. The only reason Democrats are supporting him now is because he's saying negative things about Trump.
2) He exonerated Hillary months before he interviewed key aides and Hillary herself. Then when he did get around to interviewing her they threw away FBI protocol and interviewed her at home and not under oath.
3) They continuously gave immunity to key Hillary people without getting anything in return. The only reason the FBI offers immunity is for quid pro quo (you give me this, I’ll give you that), situations, so they threw this protocol out the window as well.
4) He and Obama knew about the Russian meddling and didn't say anything because they thought Hillary would win the election, so they didn't want to stir anything up. Any complaints about how Trump is handling Russia is ridiculous since Obama (as usual), did absolutely nothing to address what Russia did now Trump’s stuck cleaning it up. Let me remind you that Obama actually lied a few weeks before the election stating there was no Russian interference when he knew there was.
5) Comey lied under oath to the intelligence committee when he said he never leaked information about the investigation. He gave confidential information to a friend who in turn, gave it to the New York Times. Someone in the private sector who lies under oath is held accountable, people in the public sector are not.
The bottom line is Comey is one of the worst FBI Directors we've ever had. He hurt both political parties with the moves he made. I ask you to listen to some of his interviews and notice how many times he speculates versus having hard facts. He speculated how Trump felt about the meddling. It's just another indicator that he didn't live by FBI protocol but instead let political bias blind him from the truth.