It's generally accepted that the decisive point in the degeneration of the Russian Revolution into totalitarianism was the death of Lenin in 1924. I would argue that this was certainly significant, but there were other events which were as significant if not moreso. The Civil War and the attempted interventions by imperial powers created the circumstances for the militarisation of Soviet society, while atrocities committed by the Red Army during the war fuelled its perpetuation and escalation by causing many former supporters of the Bolshevik regime to become disillusioned (these atrocities were no doubt themselves fuelled by more common and often worse atrocities committed by the Tsarist White Army).
Other factors causing disillusionment with the Soviet government - and increased support for insurrections against it, including the White Army - include the dissolution of the Constituent Assembly due to its domination by pro-Kerensky Right-Socialist Revolutionaries, because whether deliberately or by neglect the split in the Socialist Revolutionary Party with those who supported the October Revolution (Left-Socialist Revolutionaries) had not been accounted for on the ballot sheet, which meant votes cast in favour of the Left-SRs were counted in the Right-SRs' favour who topped the pre-split SR list; the defeat of the revolutions in Germany and other parts of the industrialised world, which caused increasing despair in the Russian cities; the frigid Russian climate which frequently caused famines, which coincided with the lack of friendly revolutionary regimes in warmer parts of the world who could have provided food aid, leading the Bolsheviks to enforce grain requisitioning policies which were massively unpopular among the peasantry; and the suppression of pro-revolutionary dissenters, including the Left-SRs, the Ukrainian anarchist Black Army, the Krondstadt rebels, and after the 1921 Congress minority factions within the Bolshevik Party.
The account of AJP Taylor in his 1964 introduction to John Reed's Ten Days That Shook The World suggests even that the Soviets never wielded true power in the "Russian Soviet Federated Socialist Republic", to the extent that the October Revolution was more the end of Soviet power than the start; that in contrast to the delegates' militancy before October, afterwards they simply deferred to the Bolsheviks. However, in light of Taylor's support of the brutal suppression of the 1956 Hungarian Revolution, it's possible he was simply trying to make the Soviets' later subordination to the CPSU seem more acceptable.