"permanent revolution"
A dictator.
Not entirely sure of the thrust of your question. The system that did such things was - and is - called "totalitarianism".
Ekaterina Gheladze Djugashvili
Fascism
he was because he was a priminister of Russia
The Great Terror or The Great Purge.
Stalin was not an emperor and did not have an empire. He was the leader for many years of the USSR
It may be called "the constant term".
Totalitarianism
That is simply called a constant.
The political system is called Totalitarianism.
Constant variable