TL;DR, if you think Comey acted inappropriately, you're a butthurt partisan hack and you've never had to make a hard decision in your life.
But he did break the rules, which is the fundamental point here. To claim that a letter to a Congressional Comittee was private and that it never entered Comey's mind that it might be leaked is utterly disingenuous. All he had to do was wait a week or so. His rationale was that there would be a political furore if it came out that he knew this information before the election. That's weak. He made assertions about the emails that were not borne out by the facts, and which took only a few more days to determine. I'm not saying he was lying; I'm saying that he spoke before he knew the facts.
For someone whom you defend as being good at making tough decisions, that's a bit of a rookie error. The most generous conclusion is that part of making tough decisions is living with their consequences, and that breaking precedent about non-interference with elections was an historically momentous decision. In hindsight, there was nothing in the actual emails that justified the decision. So he broke the rules, and he was wrong to do it. Whether he could have known in advance what the impact would be is moot. It's precisely because the results of such actions are unpredictable that Justice Department employees have a policy of simply not making statements about ongoing investigations—such as the Russia probe, for example—during an election campaign.
You can portray it as a partisan issue, but no other FBI director—not even Hoover, who was no wilting lily when it came to political shenanigans—ever actively intervened in a Presidential election. Ever.
There is strong empirical evidence that, had he not spoken up, the election could easily have gone the other way. The FBI are required to be non-partisan and apolitical. That is a fundamental precept of virtually all of the Justice Department's activities. Yes, Lynch made a mistake in allowing Bill Clinton onto the plane that day, but to revisit grammar school ethics for a moment: Two wrongs don't make a right.
I'm not butthurt, by the way. I don't even have a dog in this fight. I'm just deeply saddened that people are willing to play the same stupid fucking political games while your republic's democratic institutions circle the toilet. The point, you may recall, is not only whether Comey did right or wrong, but whether the President was right to fire him for what he did. And my point, as you might recall, is that the President was wrong to do so, even if, as the Justice Department memo claims, he handled the Clinton case in a way that undermined the integrity of the FBI as an apolitical and non-partisan organisation.
If the only way you can conceive of a differing opinion is in terms of butthurt partisanism... then I'm very sorry for you. There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy.