What are the principles of democracy that Joseph Stalin violated?
If we equate Joseph Stalin personally with the Soviet government in general during the time that he ruled the country, we can say that Stalin violated every principle of democracy that there is. He did not commit all of the violations personally, but all principles of democracy were violated by the Soviet government under his leadership.
One definition of democracy holds that it consists of three main components. These are popular sovereignty, political liberty, and political equality. That is, democracy requires that the people control the government, that people have rights, and that all people have the same amount of political power. None of these was true in the Soviet Union under Stalin.
Under Stalin, there was no popular sovereignty. The people did not get to elect their leaders. They could not vote leaders out of office or remove them in any other way. They were essentially powerless. Under Stalin, political rights were not respected. Stalin imprisoned and/or killed huge numbers of people for political dissent. There was no freedom of speech or freedom of the press under his rule. Under Stalin, people were not politically equal. Party members had much more power than those who were not in the party. They at least had some small amount of say in government while those outside the party had none.
In these ways, Stalin and the Soviet Union violated all of the principles of democracy.