Basically, this whole conversation came with an idea popping into my head. Leftists have to hear a lot about how they are the antithesis of "Western Civilization". However, we're just as valid as westerners as a lib or a CHUD. Why treat me like a foreigner in my own home country? Why do I even carry myself as a foreigner in my own country? That's because I don't think I hate western civilization.
I am simply against political forces that unjustly make others suffer for the benefit of a decadent, parasitical elite: whether it be rich people or a "desirable" ethnic/gender/etc in-group. This is neither unique to the west nor a core pillar of western civilization. However, I am accused of trying to "destroy western civilization" each time I say we should be nicer to black people or we should be more skeptical of oil companies. I don't think any of us get irrationally angry when we see the statue of David, or Ionic pillars. Plato's Republic doesn't have any passages on the importance of destroying the environment for the benefit of the wealthy, nor does it have any passage of relentlessly bullying people who have a different skin color or anyone who fucks in a way that makes the Christian God that no one is aware of yet when Plato's Republic was written upset.
However, this shows how successful the capitalists infestation of our cultural zeitgeist is: where if you criticize these oligarchs, people think you are criticizing the way of life of western civilization.
I mean it's at least implied by the notion of "Western Civilization" that Marxism is a Jewish assault on all that is good and holy. They literally kvetch about "Judeo-Bolshevism". That's why you can't take "Western Civilization" as a serious concept. If you engage with it at all you're accepting the premises of their delusional fabricated world view.
I don't know whether I fully agree or disagree with what you're saying, though I do think that the term "Western civilization" is a slippery and problematic one, and it's hard to even define a "civilization" as a distinct entity in the first place. But it's unquestionably true that Karl Marx was working from a body of thought and study that emerged out of Europe and the intellectual traditions that were particular to it. Marxism came from the West.