I wonder if that's the same one I saw before. It was called "Monster." The most notable thing about that series, is at the end they matter-of-factly state that Beria had actually poisoned Stalin, and told the rest of the Politburo, "I just saved all of you."
I just looked it up and it was called Russia's War Blood Upon the Snow. They cut Stalin no breaks in this documentary.
Was "Monster" about Stalin or more about Beria?