Why did Stalin launch the Great Terror, 1936-38?

In 1932 things looked very good for Stalin; the First Five Year Plan was complete, the peasants had been collectivised and Stalin was the 'boss' and firmly in control. However, there was still a little opposition in the Communist Party and a Party member called Ryutin circulated criticism of Stalin and how he ran the Party among the Central Committee arguing for his removal as General Secretary. Of course, Stalin was furious that someone of so …

…spurred on by Hitler's success in the 'Night of the Long Knives' in 1934, and so caused Hitler to be a threat who would want a war with Russia in the future, so Stalin introduced the Trotskyite-Zinovievite Counter-Revolutionary Plot which climaxed in the Moscow Show Trials and lead onto the Mass Purge and the Army Purge, to remove people who remembered Lenin, to produce free slave labour and to wipe out any opposition and old rivals.