It is hard to measure. Trotsky was a military leader and a visionary calling for an immediate revolution worldwide. Stalin was a behind the scenes operator who consolidated his power. European intellectuals were fascinated even by Stalin’s version, kal vehomer they might have bought the more inspiring version.
Trotsky was not more merciful, he was responsible for enough killings. When Chometz Chaim was running away, it was still pre Stalin, I think
Menu