It is his thought on religion, as nothing more than an opiate for the masses, that should concern us in this thread.
Why, his idea was to inspire a fairer society, by creating what he perceived as a fairer economic system, he saw religious belief in an afterlife and reward, as a barrier to action while we are alive.
Marx, Lenin and Engels inspired the Communist Soviet state which included prohibitions against religion and, although those prohibitions have eased somewhat, they still exist.
Firstly they knew that throughout Europe, state religions like Christianity had endorsed the divine right of the Tsars as absolute rulers. The people lived suffered and died in a totalitarian state, and religions endorsed and benefited from this, so it's hardly surprising the revolutionary powers distrusted those religions. They have not "eased somewhat" there has been a massive upturn in religion, and in religious intolerance in the former Soviet countries.
The poster asked for a dangerous atheist thought. I think Marx's thought qualifies.
Except your rationale seems to be it was the root cause behind the state atheism of the former Soviet Union, but it wasn't, and it certainly wasn't what Marx was saying. I think he made a very salient point, that throughout human history, religious promises of a posthumous reward, have stilted people's desire to fight for a fairer existence right here right now. I don't think you've understood what Marx was saying, and it certainly had little if anything to do with the Soviet state ostensibly prohibiting religion. Stalin cynically funded the Russian Orthodox church during WWII, in order to help motivate all the people to fight fascism.
Marxism is an economic ideal that doesn't really work, it has little directly to do with atheism, which is simply a lack of belief in any deity. The suppression of religion had more to do with trying to end the idea of divine right of kings, and of course Stalin's paranoia could not have allowed any organisation to rival the state machine, as he wanted absolute power. Since atheism has no dogma or doctrine and atheists are not an homogenous group in the way Christians were, the rationale quite clearly was that they would be easier to control and manipulate.
Now for the sake of argument let's say all that isn't true, your idea doesn't seem to have any relevance to the accusations levelled at new atheists, as they are not advocating for a communist or a totalitarian state, and though they might hope for a secular government, that only means the separation of church and state, which we still don't have in the UK.