Karl Kautsky

Known as the "pope of Marxism", Kautsky wrote the SPD's 1891 Erfurt Programme and defended orthodoxy against Bernstein, insisting capitalism must give way to socialism. Then he turned on Lenin: The Dictatorship of the Proletariat (1918) argued that socialism without democracy and a free parliament was not socialism at all. Lenin called him a renegade, which makes him the best guide to why democratic Marxists broke with Bolshevism.