One day I will need to take the time to real all of Marx to understand how he ostensibly transitioned from a correct statement of reality in the Preface to such a horrific killing field of Communism.
He did not, by and large. At least not in the way that Communism is understood today. Communism, for him, was just a philosophical concept, some kind of evolutionary (end?-) point of humanity in the future that would happen naturally (tribes -> feudalism -> capitalism -> communism) (see
https://en.wikipedia.org/wiki/Dialectical_materialism). The utopian kind communism is not an authoritarian system, it's rather that people would voluntary follow the lifestyle of *from each according to their ability, to each according to their needs*, as they would finally realize they can be freed from the coercion of capital, money, and property. Essentially a world where the provision of all basic needs (and beyond) is automated by robots/computers anyway, and workers have to contribute very little, if at all.
The discourse on the left is rather about what needs to happen on the road to that goal. Some kind of consensus is that changes first and foremost happen through revolutions. The right strategies have to be employed to steer revolutions (that would necessarily happen in the course of history anyway) into the right direction. Although Communism means a society without both capital and the state, some revolutionary thinkers wanted to use the state as a temporary(!) tool, and subvert it by turning the dictatorship of the bourgeois into the dictatorship of the proletariat; that was Lenin's line of thought, and he got his way, the rest is history.