Yup, that's why I've always recommended (along with relevent cold war liberal and neo-con texts!;-) Trotskyist and other critical marxist works like, "The Communist Movement, " by Fernando Claudin, Monthly Review Press, late 70's, "Stalin and the European Communists, " by PCI historian, Paulo Spriono, Verso Books, "The Stalinist Legacy, " ed. by Tariq Ali, " F. Furet, "Communism ... The Passing of an Illusion: The Idea of Communism in the Twentieth Century, "Roy Medvedev, "Let History Judge: The Origins and Consequences of Stalinism, " , Rudolf Bahro, "The Alternative in Eastern Europe, " Verso Books...
All these any many more detail the horrendous repression by Stalinists of communist and socialist oppositions. (And many, not even remotely oppositional, esp. at the height of the purges in '37.) Michael Pugliese