You are, of course, free to say that the murderous regimes that have popped up again and again are not the true Marxism. But then you have to explain why the false Marxism has appeared so often, and then to explain, given that millions of lives could be at stake, how to prevent a recurrence.
That's only a small piece of vanguardism, which while it is a big change isn't the only significant change (and is, in fact, an adaptation to the fundamental change, which is abandoning the prerequisite of the development of a broad working class identity under developed capitalism.)
> You are, of course, free to say that the murderous regimes that have popped up again and again are not the true Marxism.
Well, I mean its self-evident true, and I didn't need your permission, but thanks.
> But then you have to explain why the false Marxism has appeared so often
I don't see why I should, since I am not advocating Marxism and, even if I was, the reason a system designed to be workable and leverage existing dissatisfaction in undeveloped, precapitalist conditions keeps doing so is neither nonobvious nor germane to discussing the merits of a different system in the developed world. Leninism has done horrible things in the environments which it was carefully crafted to thrive in, but even with the assist given by it's opponents fear mongering equivalence-drawing that painted all opposition to capitalism as equivalent to Leninism, it's done stunningly poorly at gaining any traction in places that don't have he conditions for which it was adapted from Marxism.
That’s the problem with Leninism.
The (biggest; there are others but this prevents even useful discussion of those) problem with Marxism is that people (even those who oppose Leninism and disbelieve everything else associated with it) accept Leninism’s propaganda that it is Marxism, rather than a system which adopts the rhetoric and stated end goals of Marxism, but abandons both it's mechanisms and explicit preconditions.