Nikolai Yezhov
He joined the Bolsheviks in April 1917, led by Lenin a few months before the October Revolution. He was known as a determined loyalist of Joseph Stalin, and in 1935 he wrote a paper in which he argued that political opposition must eventually lead to violence and terrorism; this became in part the ideological basis of the Purges. He became head of the NKVD in early 1937, after the dismissal of Genrikh Yagoda. Under Yezhov, the purges reached their height, with roughly half of the Soviet political and military establishment being exiled or shot, along with hundreds of thousands of others, suspected of disloyalty or wrecking.