In his review of Alfred Rosmer's book 'Trotsky and the Origins of Trotskyism' (February SR), John Molyneux says the argument about Zinoviev's role in the Comintern is 'obscure', and that Stalinism is explained by 'objective factors'.
Certainly the rise of Stalin was not just the defeat of 'good' individuals by 'bad'. But the isolation of Russia was not inevitable--it was caused by the failure of the Comintern, and here Zinoviev's role was important. There are two examples of this.
At the second congress (1920) there were a number of delegates from anarchist and syndicalist backgrounds. Lenin and Trotsky welcomed them, stressing unity in action. Zinoviev addressed them in terms that were patronising, sectarian and insulting. The debate is quite relevant to tactics in the anti-capitalist movement today.