1. If you've seen a self-proclaimed Marxist say that the state is wholly negative, they were not, in fact, a Marxist, but most likely an anarchist.
2. Stalin being "one of history's most brutal dictators" is imperialist drivel; even internal CIA documents stated that the USSR had collective leadership. If you're talking about the leadership as a whole, there are certainly things to criticize -- there were undoubtedly excess imprisonments and executions during wartime, because they were defending humanity against fascism, there were fascist collaborators in various republics (such as Solzhenitsyn, the liberal darling), and the survival of the USSR was at stake, but such excess could be found in many other countries (perhaps most prominently the concentration camps for Japanese people in the US, whose survival was very much not at stake). This propaganda has not been accepted by many countries in the Global South, and I would challenge you to provide some actual evidence (on par with that provided for Nazi Germany, the US, Israel, or other fascist regimes).

I apologise if I have misunderstood the text

Then I suggest you read further through the link I provided, and furthermore educate yourself on what Marxists actually believe, before accusing them.

when one of the crucial figures of your ideology is known as one of history's most brutal dictators.

I dismiss this claim whether you are referring to Lenin or Stalin. In both cases there's ample evidence to the contrary. This perception is mostly formed by Western propaganda.

that you say that authority and the State is wholly negative

Anarchists say those sorts of things. Not me. Yes the goal of communism is to EVENTUALLY create a stateless society. But to do that, we first need to a) hold a revolution, b) supress counter-revolution, c) be able to defend the revolution from external enemies, and d) eliminate the bourgeoise as a class.

That last part doesn't presuppose we eliminate them by killing them necessarily, but we need to curb their immense political and economic power, and to do that we need a state that can actually stop them from creating a counter-revolution. In the exact same vein, I'll reuse the example of the French revolution, which eliminated the aristocracy as a class, so as to be able to actually found a democratic nation.

And of course the other function of the state is to be able to defend itself from external threats. To do that we need the most "authoritarian" structure of all, i.e. a functional army, with a functional industrial capacity behind it to support it.

We can sit back now and say that the Soviet Union was an "authoritarian" state, but we shouldn't forget what it went through to survive. In its first 30 years of history, the Soviet Union went through 2 world wars (both devastating for it), 2 revolutions, a civil war (which had the added component of 8 major powers invading at the same time), 2 other wars, and a full economic blockade, and all that while trying to transform itself from a feudal empire into a socialist industrialized democracy. Without a state to protect itself, the Soviet Union would be a footnote in the history books right now, much like the Paris Commune and the Spanish Republic.