Depends on your favorite measure for atrocities, there was a civil war in Russia, there were the Stalinist purges all over the Soviet Union, there was the colon

Depends on your favorite measure for atrocities, there was a civil war in Russia, there were the Stalinist purges all over the Soviet Union, there was the colonization of the Asian part of Russia and there was Holdomor. Each of these are among the worst human rights abuses, but actually I am not sure if you can really argues which one of them was worse.