Сталин и его маршал

Why did Stalin destroy the leaders of the Red Army before WWII? Was or was not the so called "case of Tukhachevskii" a consequence of Stalin's sick and despotic mind? This biography of the leading military leader Mikhail Tukhachevskii (1893-1937) and history of the so called Tukhachevskii plot is written by a well known Russian military historian.