Stalin's own archives and the memoirs of Stalin's own Politburo members all agree that Stalin killed millions of people. Even at the time, Pravda reported on the political executions and arrests.

And I would call any book about Stalin and Hitler that doesn't depict both of them as megalomaniac mass killers the work of a lying hack.

So no, I don't think Montefiore's book is an "agreeable fiction." The dozens of books you've read about Stalin, however, clearly were.