It was on December 1st, 1934, that a gunman burst into the offices of Sergei Kirov, the Mayor of Leningrad (now St Petersburg) and shot him dead. Stalin used the assassination as a pretext to launch his Great Purge of dissident Communist officials and any party members who did not toe Stalin's official line one hundred percent. In the show trials that took place in the late 1930s, over a million people were put to death after a sham judicial process.
Although Kirov was loyal to Stalin, popular opinion at the time, supported by some later historians, was convinced that Stalin had personally ordered the murder fearing that Kirov's popularity was making him a potential rival to Stalin himself. When Kirov had been elected to the central committee earlier that year, he had only three votes against, the fewest of any candidate, while Stalin had 292 votes against.
What was new about the Great Purge that Stalin now instigated was that, for the first time, members of the ruling Communist Party featured prominently among its victims. Most held party offices of some kind, but the purge of the party was accompanied by a purge of society. Many Bolsheviks, famous for their roles in the 1917 Revolution, were seized and convicted in show trials, expelled from the party and then executed. Many of them "confessed" to being involved in Kirov's murder.
Western left-wing journalists, of the type Lenin had called "useful idiots," covered the trials and reported them as open and fair. What they did not know then, but what is now known, were the methods used to extract the confessions on which the guilty verdicts depended. They included torture by repeated beatings, simulated drownings, forcing prisoners to remain standing, depriving them of sleep for several days, and threatening to arrest and execute their families.
The whole process was brilliantly conveyed in Arthur Koestler's "Darkness at Noon," which ends with Rubashov, the fictional Old Bolshevik, making a full and public confession after torture to crimes he did not commit, and doing so out of a lingering loyalty to the party. The book ends with his execution.
A vast series of closed trials was held in addition to the show trials. These were of top Soviet military leaders, tried in 1937-38, and executed. This was accompanied by a massive purge of the Soviet armed forces, an event which helps to explain the initial poor performance of Soviet defending forces when the Germans invaded in 1941. Their leadership had been eliminated, and their morale was low in consequence. It took the anvil of war to renew both of these.
Many dissidents opposed to Stalin who had fled abroad were hunted down and murdered by killer squads sent to eliminate them. Leon Trotsky was killed in Mexico. No-one was safe. Robert Conquest refers to this period as "The Great Terror," (the title of his book on the subject), deliberately calling to mind the Reign of Terror that gripped the French Revolution.
It is possible that Stalin was seeking to expunge all forms of Communist thinking except for his own narrow party line, but there is a good case for supposing that he was by then totally paranoid, corrupted by the absolute power that has always characterized communist regimes. The leaders think the cause is so right that it justifies anything to sustain it, even brutality, torture, and mass murder.