Most of the deaths I'm aware of were the result of famines, which I already explained. There was also some political repression, especially with Lenin's red terror and the Russian Civil War. Still, Stalinism was the most horrific period for the Soviet Union, and for communists and the working peoples' movement everywhere.
Workers owning the means of production changes everything. It establishes true democracy -- the ability to make decisions that immediately affect your own life, right there in the workplace, and to produce according to need instead of profit (where many goods go unsold or are destroyed).
Socialism requires true democracy, and in communist one-party dictatorships, that is impossible. "Democratic centralism" is a farce. You should also be aware of different schools of communist thought, like Trotskyism, following the ideas of Trotsky who declared the Soviet Union under Stalin to be a "degenerated worker's state", the left-communists (responsible for the significant Kronstadt rebellion, and resistance against the USSR and Nazis in Germany), many of whom are council communists and of course anarchist communists and syndicalists (libertarian Spain, and Makhnovia). I consider myself an anarcho-communist.