Sorry, not seeing your point here.

If Very Bad Things happen every time a particular idea is put into practice, it's a bad idea, regardless of whether the Very Bad Things were explicitly mentioned when the idea was put forth.

More to the point, where did Marx and Engels write about narrow vanguardism instead of broad class consciousness, bypassing capitalism rather than moving forward from it, or any of the other central points of Leninism, which (and not Marxism) is the common ideology shared by the people responsible for those 100 million dead bodies?

The only people trying to put Marxism, rather than Leninism, into effect have been in developed capitalist (and now, in part due to their efforts, post-capitalist mixed) economies.