That quote is extremely hinged on context in which it was made, and it would serve you well to internalise that context before throwing this quote around pretending it to have been something Marx lived by.

Correct, Marx wasn't just randomly terrorizing people. He was referring to the Proletariat making no apologies for revolution and taking up arms against the bourgeoisie and their enablers, something Lenin and the people of the USSR carried into reality. Lenin descibed what you're doing to Marx and Engels right now quite well:

"What is now happening to Marx's teaching has, in the course of history, happened repeatedly to the teachings of revolutionary thinkers and leaders of oppressed classes struggling for emancipation. During the lifetime of great revolutionaries, the oppressing classes constantly hounded them, received their teachings with the most savage malice, the most furious hatred and the most unscrupulous campaigns of lies and slander. After their death, attempts are made to convert them into harmless icons, to canonize them, so to say, and to surround their names with a certain halo for the "consolation" of the oppressed classes and with the object of duping the latter, while at the same time emasculating the essence of the revolutionary teaching, blunting its revolutionary edge and vulgarizing it. At the present time, the bourgeoisie and the opportunists within the working-class movement concur in this "doctoring" of Marxism. They omit, obliterate and distort the revolutionary side of this teaching, its revolutionary soul. They push to the foreground and extol what is or seems acceptable to the bourgeoisie. All the social-chauvinists are now "Marxists" (don't laugh!). And more and more frequently, German bourgeois scholars, but yesterday specialists in the annihilation of Marxism, are speaking of the "national-German" Marx, who, they aver, educated the workers' unions which are so splendidly organized for the purpose of conducting a predatory war!"

• Vladimir Lenin, The State and Revolution

That was not my claim, but thank you for so generously misinterpreting what I said. Lenin implemented the violent oppression of dissenters and opposition in a socialist system. That was carried further by Stalin, under whom ‘counter-revolutionary’ became an extremely malleable term that could mean anything not fully aligned with his ideas. The fact that you think political violence and terror is a core tenet of Marxism tells me that you’re the one who might need to brush up on their history a little bit.

Lenin implemented the world's first Socialist state, and this state violently oppressed the bourgeoisie, fascists, the White Army, rebels, and so forth. The fact is, political violence is often sadly necessary against those who would crush the Socialist state, like the 14 Capitalist countries that jointly invaded the USSR after its founding. A Marxist project that rolls over and dies the second fascists and the bourgeoisie fight against it isn't Marxist. Blackshirts and Reds is a good quick read on the tangible benefits AES states achieved despite brutal opposition from the outside.

In fact, authoritarian socialism - as practiced in virtually every single Marxist-Leninist country that ever existed - was completely counter to the ideals of Marx and Engels. The people we have to thank for creating the violent authoritarianism that pervaded communist countries in practice are Lenin and Stalin. “Dictatorship of the proletariat” may have been a phrase used by Marx, but he never fully elaborated on what that should or could look like. And fascism as created by Mussolini and unleashed upon the world by Hitler didn’t even exist during Marx’s lifetime. Even Marx’s views on religion were a lot more complex and multifaceted than what Marxist-Leninist governments turned them into.

This is nonsense. First of all, what separates "authoritarian" Socialism from "non-authoritarian" socialism? All Marxist-Leninist states practice democracy and allow more participation in the way society is run than Capitalist states for the average person. Soviet Democracy by Pat Sloan is a good resource on this. Secondly, the idea that Marx and Engels never had a clear idea of what the Dictatorship of the Proletariat would look like is further nonsense - Marx described the Paris Commune as the first implementation of the DotP in reality. Marx and Engels knew quite well that violent suppression of bourgeois elements was required.

Furthermore, whether Marx or Engels really observed fascism is utterly irrelevant, unless your point is that they would not have been anti-fascist, which is nonsense.

I don’t know if either of you have ever lived in a Marxist-Leninist country (as in lived, not just visited). I was born in one. I lived in another for five years. I’ve seen the before and after, first-hand. That’s my pedestal. How’s the weather up there on yours?

A non-sequitor. Spending early childhood in an AES state does not mean you know how it works, nor what it deals with on a daily basis. Even people who live their entire lives in Capitalist states go without knowing how they function.