That's different from saying that what Marx envisioned as a utopia (or even the path to get to utopia) was what the Soviet Union became. He did not foresee that nightmare and he did not want that nightmare, and I genuinely believe that most Marxists don't either. They are, if anything, a bit too well meaning and naive, and think that everything will turn out for the best once the proletariat gain control over the means of production.
I'm not sure I agree with you about human nature, but I do think an army of unscrupulous people stand ready and willing to subvert that sort of system and to use true believers to further their own welfare rather than the welfare of society as a whole.
True believers willing to kill others to build their utopia appear all throughout history, not just in ostensibly Marxist or Communist societies, but in the Inquisition, the Crusades, Nazi Germany, and many other horrific places and times, and they never end well (even if you think some ideal ends justify such bloody means).
If a true utopia is ever to arise (if that's even possible), it has to do so and be sustained through non-violent means.
You can't radically change people's way of life without doing all of these things. So it's implicit. Western liberals especially find this very hard to acknowledge. Lenin had no such misunderstanding: dude was perfectly fine with mass murdering peasants and starving children to death. The ends, in his mind, justified the means.
Marx's writings were also descriptive, not prescriptive. He imagined how things could be if everything went perfectly, rather than gave you a concrete set of steps to get there. The problem is with the latter, not the former. It's a "sperical cow in a vacuum" kind of thinking, very black and white, not in the least suitable for the real world, and at odds with human nature.