I think he is proving to be a cunning political operator for the Republicans.
What did the Trumpers hate him for? Not charging Hillary. But what would that have achieved? It would have achieved the one thing that Hillary couldn't: passion. It would have inflamed the Democrats, and united them behind her. They were confused. They were squabbling. They were confident she was going to be their choice, but that confidence was being tested. Even Democrats were wondering for a bit, "Can Hillary even beat Bernie? Should she?" That question would have disappeared overnight if the Democrats felt a Republican intelligence chief was sabotaging their chance at the White House, and for what? Do we really believe it would have hurt her to be "under investigation" for something that everybody already knew she did? Especially when Pence was doing the same thing-- just "legally" (so it's totally okay, and only evil when a Clinton does it).
He comes out a week before the election and wounds Clinton mortally with a bizarre announcement about Abedin and her husband's laptop that really had nothing to do with Hillary, at all. What's more...it appears there is a good chance he knew that when he made the statement.
Now, with all of this "#resist" shit lingering, and the Democrats throwing tantrums daily since he won, he's cemented all of that anti-Trump fury, and all of its most potent attacks against him, into this one issue: Russia. What do we know about Russia? That both Morell and Clapper said our intelligence agencies were aware of no provable links to Russian government officials. There is so much smoke, here, that obviously something is rotten in the state of Denmark, but it seems quite credible that none of it will stick to Trump.
So what has Comey achieved-- if I'm correct and he's doing everything he can to aid his own party? He has seized authority over the narrative. He has made this trial the stage for whether or not Trump is a corrupt man. Of course, if Trump is innocent of the first, he isn't innocent of the second, but that isn't how public perception works. Just as sometime merely being on trial is enough to stain you with the suspicion of guilt, sometimes clearing someone of one charge is enough to clear them of the perception of guilt of a related charge (or many charges) for which they were not actually exonerated.
The man appears to be possibly the most brilliant political operator we have on our American stage today. Of course, none of the above applies Occam's Razor. This borders on a CT strategy of thinking. The simplest answer appears to be that he is true Lawman and he simply wants to maintain some semblance of integrity within his own sector of government.