Well, there were multiple reasons, and that all depended on what you were to Stalin.
Stalin was in charge of a highly militarized police state, the new head of the radical revolutionaries that had given itself great powers in order to safeguard their revolution. Any romantic figure from the Bolshevik October Revolution could become the new soul of the Party, any admired Red Army general could pose a competent menace, any network of bureaucrats and functionaries could use their power to stifle Stalin’s vision, so they had to be purged to ensure no one could threaten Stalin and his vision of the single socialist state behind the single socialist leader with the single socialist vision. Stalin’s Great Purge was meant to eliminate any that could pose a threat to the paranoid leader.
Then, there was the war against the kulaks. In Russia, the kulaks, the wealthy peasantry, were incredibly influential on food production. These peasants had become wealthy thanks to the Stolypin reforms of 1906. Stolypin’s idea was simple, he believed that if peasants owned their farms, they would become more productive small businesses hiring employees. This would mean higher quantities of food produced, lowering the price, invest the kulaks in the success of the Tsarist regime, and lower radicalization with the subsequent lower price of food, higher employment rate of peasants, and directing productive activity toward competing in a free market. Stalin labeled the kulaks as class traitors and enemies to socialism and the revolution. This led to the death of roughly five to six million kulaks as estimated by Solzhenitsyn, and forcibly seized the land into the kolkhoz and sovkhoz, the collective and state-run farms.
There was simply the incompetence in Stalin’s forced collectivization policies.The loss of so much intellectual capital and disruption in agriculture, along with the resulting inefficiency and corruption of the collective farms, resulted in large grain shortages and required the sale of Siberian precious metals for grain imports. The poor use of the resources that the USSR had led to famine and ecological disaster (which in turn, has its own impact on agricultural productivity). In addition to the kulaks, any who did not meet their quota or refused to turn over supplies were deemed enemies and either sent to corrective labor camps or executed.
Then, there was the rapid push for industrialization. The seizure of private assets was not enough to transform the Soviet state to a fully industrialized one, and Stalin was convinced he had to fully industrialize. Stalin was convinced that eventually he would have to lead a great war against the capitalist west to impose communism, and to do that, he needed industry and tanks. So he pushed for a greater push for “production of the means of production,” as it was referred to in Soviet literature. So, much of Soviet production was used to fund the five year plans that Stalin embarked for the rapid industrialization, which included much of the remaining food stores.
So, Stalin killed some because he perceived them to be threats to him, some because he believed them to be threats to his vision, some because he named them enemies to socialism, and some were simply human costs in his push for industrialization.
Thanks for the question, Anon.
SomethingLikeALawyer, Hand of the King