For non- or anti-socialists, it is important to cojoin and essentialize socialism in genreal to the specific socio-cultural and historical trajectory of the Soviet Union. Demonizing Stalin as an individual then exposes and demonizes the 'truth' of EVERY POSSIBLE socialism.
For many on the left, on the other hand, the only way to save the idea of socialism is to dissasociate itself from what happened in the Soviet Union as State Capitalism, Tsarism in Communist clothing, or a mirror of Fascism, and the only way to saintify Lenin and the ‘October’ Revolution is to demonize Stalin as the individual who drove the revolution off its rails.
Bryan