communists will tell you
the soviet ideology wasn't
communism.
This falls in no-true-Scotsman territory, and should be seen as a self-exculpatory narrative, after all, they sent millions to an early death, yet the societies they created were worse than those that they overturned, and much worse than the best functioning societies at the time.
Let's also be clear that Soviets read the works by Marx and Engels in great detail, it's not like they didn't know what Marx and Engels wrote. For example,
the Communist Manifesto [1, 2] outlines a few essential demands
[3] for a transition that will eventually lead to communist utopia.
Every single one of those was executed, and achieved by Stalin.
Exactly what "communism" should mean has never really been clarified,
but whatever Marx and Engels meant by it, the fact that carrying out
their 10 key demands exacted an 8 digit death toll is an extremely
strong indication that there is something fundamentally wrong with
Marx and Engels ideas.
Moreover, the world A/B tested this: two countries were split (Germany
and Korea), one was ruled by a communist party (Soviet aligned) the
other capitalist and within a decade the capitalist one was doing
dramatically better than the other. So much so, that you Soviet
aligned one had to turn itself to a prison to prevent the population
from fleeing. (Yes I am aware that this is not a repeatable scientific
experiment. However it is the most decisive 'experiment' you can hope
for in politics and social theory. It is what we must learn from. To quote Marx: "A Nation should and can learn from other nations".)
[1] K. Marx, F. Engels, Manifesto of the Communist
Party. http://activistmanifesto.org/assets/original-communist-manif...
[2] https://en.wikipedia.org/wiki/The_Communist_Manifesto
[3] Briefly (and from [2]): progressive income tax; abolition of
inheritances and private property; abolition of child labour; free
public education; nationalisation of the means of transport and
communication; centralisation of credit via a national bank; expansion
of publicly owned land, etc.—the implementation of which would result
in the precursor to a stateless and classless society.