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.
Also in the early 1920s Zinoviev tried to 'Bolshevise' the parties of the Comintern. This meant imposing a single model of the 'Leninist' party. This was a myth--as Tony Cliff shows in his book Lenin. There were at least seven different versions of Bolshevism, according to circumstances. In his final speech to the Comintern Lenin attacked this nonsense and told foreign Communists to think for themselves--again very relevant to current discussions on organisation.
Individuals matter, and tactics and organisation matter. That is why revolutionaries must study history, even the apparently obscure point.