Soviet archival evidence shows that the purges were democratically initiated by the party at large and were initially very popular due to the real problem of capitalist sabotage They got worse when the NKVD became overzealous and stepped beyond the bounds that were given them. The evidence shows that Stalin actually spent more time trying to reign in the excesses, but was often unsuccessful largely due to the weakness of the Soviet state that was recovering from a brutal civil war.
Of course, the purges were very excessive and rightly considered one of the big mistakes of the Stalin era, but the idea that they were his is ahistorical.