Comey would have conferred a big advantage on Trump if he had decided to recommend prosecuting Clinton -- a decision that, in my view, would easily have been defensible.
Comey would have conferred a big advantage on Clinton if, having decided against prosecuting her, he had declined to explain the basis of his decision to the American people and to Congress, and then refused to meet his promise to advise Congress if the FBI re-opened its investigation. A refusal to explain would have created the false impression that Clinton had not acted with great carelessness.
By taking a middle course -- not prosecuting but being transparent -- Comey probably came as close as he could have to not tilting the election in favor of either candidate. This doesn't mean he acted properly. It does suggest that, if one recognizes the full context of Comey's actions and the complexity of the situation, they are not really inconsistent with the "don't help or hurt a candidate" tradition he is accused of violating.