Stalin's paranoid killing-off 30,000 Soviet military officers prior to WWII actually prolonged the war!
Most certainly, but that also has to be balanced by the fact that without Stalin's ruthless five year plans forcing through industrialisation, the USSR wouldn't have been in any sort of position to equip and maintain an army capable of halting the Germans at the gates of Moscow, Leningrad and Stalingrad....
I would agree with that. He realized very quickly during the Winter War that offing so many of his military leadership was a devastatingly bad mistake. It allowed the Finns to hold off a much larger Soviet force for 4 months in what on paper should have been a very quick and decisive conquest for the Soviets.