Who was Joseph Stalin's scapegoat, and who was his private police force and how did he use them?

Sergey Mironovich Kirov

(born March 27, 1886, Urzhum, Vyatka province, Russia — died Dec. 1, 1934, Leningrad, Russia, U.S.S.R.) Soviet political leader. After joining the Bolsheviks, he extended the Communist Party's control in Transcaucasia, and in 1926 Joseph Stalin appointed him head of the Leningrad party organization (1926). He modernized the city's industries, was elected to the Politburo (1930), and acquired power that nearly rivaled Stalin's. In 1934 he was assassinated by a young party member, Leonid Nikolayev, who was later shot, along with 13 suspected accomplices. Stalin, claiming that a widespread conspiracy of anti-Stalinist communists planned to assassinate the entire Soviet leadership, used the assassination as a pretext to institute the Purge trials. In 1956 Nikita Khrushchev suggested that Stalin had engineered Kirov's assassination.

http://www.answers.com/topic/sergei-kirov

Stalin had several scapegoats. One of the earliest and best known was Trotsky. The purges referred to above lasted from about 1935-38. The entire Bolshevik 'Old Guard' was ruthlessly purged off. Stalin also ordered the secret police to frame some innocent people - on order to spread terror.

In January 1939 Lavrenti Beria was appointed Head of the NKVD (secret police) with orders to bring the purges to an end. His predecessor had just been convicted of "sabotage" by overfilling the labour camps in Siberia.

https://en.wikipedia.org/wiki/Lavrenty_Beria