A revolution is certainly the most authoritarian thing there is; it is the act whereby one part of the population imposes its will upon the other part by means of rifles, bayonets and cannon — authoritarian means, if such there be at all; and if the victorious party does not want to have fought in vain, it must maintain this rule by means of the terror which its arms inspire in the reactionists. Would the Paris Commune have lasted a single day if it had not made use of this authority of the armed people against the bourgeois? Should we not, on the contrary, reproach it for not having used it freely enough?

Frederick Engels, On Authority

[The workers] must work to ensure that the immediate revolutionary excitement is not suddenly suppressed after the victory. On the contrary, it must be sustained as long as possible. Far from opposing the so-called excesses – instances of popular vengeance against hated individuals or against public buildings with which hateful memories are associated – the workers’ party must not only tolerate these actions but must even give them direction.

Marx & Engels, Address of the Central Committee

An interesting read for you would be Leon Trotsky's The Revolution Betrayed. Trotsky was Lenin's right hand man and designated successor so he has a particular insight into Stalin as a man and a Bolshevik.

Similarly, the biographies The Prophet on Trotsky by Isaac Deutscher will give a more detailed view of things.

I would say that the Soviet experiment was doomed to fail the moment the Bolsheviks suspended any democratic rights from below in their establishment of the dictatorship of the proletariat. After WW1 and the civil war the already small working class was almost obliterated. The Bolsheviks made a conscious decision to have the Party make all decisions amongst themselves until the proletariat was capable of carrying out elections when it grew to an acceptable size again. Following Stalin's purges of the Old Guard there really was no one left that both knew of and believed in that original vision so that temporary rule by the Party became quite permanent.

I'm not as well read regarding the Maoist experience in China but hopefully I've given you some insight to the Soviet side of things.