Stalin was a considered a failure due to mass killings and using fear to control the people.However, he increase the production rate of the defense structures as he knew that Russia could not withstand an attack from the other countries. With the increase of the number of weapons/defensive structures, he was not only able to defend Russia, but also became one of the winning countries in World war 2.

While he did increase the production rates in such a short time, the increment could not be sustained in long term, and the increased production was of basic necessities and defenses. There was hardly any production of consumer goods.

Basically, the people are alive, but have nothing to do but work

(Lionel)

When Stalin introduced collectivization, many people(peasants/farmers) resisted by killing their animals and burning their crops. This lead to a shortage of food production. Some of those who resisted were gulags(concentration camps) to be killed. After that, there was a natural disaster which was a famine. The lack of food led to many people turning to cannibalism to survive. When Stalin introduced collectivization, he killed those who resisted, which was in millions, but the cannibalism in the famine killed a lot more. So, in other words, by introducing collectivization, he killed millions, which lead to the cannibalization of even more people. The death count of collectivization has a greater impact than the promised (at that point of time) rewards of collectivization that Stalin described