He apparently thought she would win, and that if the existence of all these emails became known, or turned out to be material, that would cast a shadow on her win, on the FBI and Justice Dept, and yes, on him too. The Russia investigation was just getting started, had little more than uncorroborated evidence against a small number of Americans (not Trump), and so he felt that would be premature.

I don't find his characterization particularly hard to believe. It's way easier to judge his actions in retrospect.

Dems will forever blame him for costing Hill the election (not provable but possible), and Trumpies will never forgive him for leaking and helping bring a special counsel, and so I think he is, in a way, just flipping his bird at everyone and laughing all the way to the bank.