Well.. Some documentaries are biased others are not. There is no doubt that the Soviets behaved badly in Germany, albeit they were extremely merciful, if you consider the "you reap what you sow" word of wisdom.
Basically, what this documentary does is exaggerating and making claims (like the casualty figures, completely unsourced and without substance). In truth ~ 10 million people (and only 8 million "proven") died due to Stalin's rule. Vast majority (85%+) were due to starvations, which can not solely be attributed to the massive taxation and incompetence, but also to self-destroying actions by the Kulaks (burning their crops, slaughtering farm animals) as well as droughts.
I'm giving it a 1 because some of the interviews were legit, but it does not show the grand picture but instead tries to distort it.
Final point: Stalin was an average, human-less dictator. He just happened to rule a very big country. Though in the end, it could be argued that his brutality united the country, and him treating people as a resource, created enough industrial power to defeat Nazi Germany.