By the time Kotkin is finished with this first volume, the Stalin that he produces is an “uncanny fusion of zealous Marxist convictions and great-power sensibilities, of sociopathic tendencies and exceptional diligence and resolve”.
Stephen Kotkin wrote a biography of Joseph Stalin. The sentence you quote is a review of the book.
Imagine yourself standing behind the shoulder of an artist who is painting a picture of a woman. You can see what he is doing with the canvas. At first you see just some lines, then some dabs of color, and then gradually it merges into an image. You can then say to a friend:
I stood and watched an artist paint a woman's portait. I could not make out much at first. Then I saw some similarity with the model. By the time he was finished with the painting, the woman that he produced on the canvas had a very romantic air about her.
In the same vein, the author of the review did not know what an image of Stalin would emerge in the book. The reviewer started reading. At first it was just a variety of historic facts about Stalin. Then the reader gradually discovered that the various facts mentioned by Kotkin (like dabs of colors on a canvas) merged into an image of Stalin. What kind of image?
an “uncanny fusion of zealous Marxist convictions and great-power sensibilities, of sociopathic tendencies and exceptional diligence and resolve”.
It's as if the reviewer stood behind Kotkin's shoulder and watched him gradually produce an image of Stalin. And the reviewer now relates this experience to the reader.