In the late 1930s Stalin's paranoia about being the greatest and unopposed autocrat led him to carry out the most vicious and extensive extermination of anyone who might be considered an enemy or opposing influence, and very many more besides. After Lenin's death, Stalin had had to share power with two others.
He was no team player and he vowed he wouldn't be in that position again, therefore his purges or reign of terror. He wanted to remove Leninists, Trotskyists, any peasants with money, any business dealers, anyone with potential influence. Many were killed, and nobody knows the exact number.
Numerous people simply disappeared and Stalin's men covered their tracks well. The official death toll is unknown, but there were 1,548,366 detained persons, of whom 681,692 were shot. Some days more than 1,000 executions were carried out, and this terror went on and on. Various historians claim that the real number of victims could be twice that Russia accepts as the death toll.