The Soviet anti-religion campaigns included plenty of mass murder.
…When church leaders demanded freedom of religion under the constitution, the Communists responded with terror. They murdered the metropolitan of Kiev and executed twenty-eight bishops and 6,775 priests…
…The sixth sector of the OGPU, led by Yevgeny Tuchkov, began aggressively arresting and executing bishops, priests, and devout worshipers, such as Metropolitan Veniamin in Petrograd in 1922…Archbishop Andronik of Perm, who worked as a missionary in Japan, was buried alive. Bishop Germogen of Tobolsk, who voluntarily accompanied the czar into exile, was strapped to the paddlewheel of a streamboat and mangled by the rotating blades…
…During the purges of 1937 and 1938, church documents record that 168,300 Russian Orthodox clergy were arrested. Of these, over 100,000 were shot…
Wikipedia – Persecution of Christians in the Soviet Union