When the Democrats called for his head after the election, the investigation of Clinton’s e-mails was completed. Obama could have fired him in October but refused to do so despite calls for it, knowing full well that it would be a blatant case of partisan meddling and obstruction of justice to boot. Trump fired him in the middle of an investigation into his campaign.
Forget about whether Comey was credible, that’s not the issue. The serious problems here are the timing and the administration’s admission that they fired him to make the investigation end, the sooner the better. Few people are crying foul over Comey himself, they’re infuriated at the blatant, likely illegal attempt at sabotaging an investigation justified by rationale that has been changing every couple of hours since the announcement.