• >No Communist Dictatorships casually committing genocide on 20-60 Million of their own people.
• >The Nazis might not exist, as they were initially a reaction to Communism.
• >No Red Scares or Communist Boogeymen.
• >No Far-Right Military Juntas established to "Protect the country from Communism."

Wait, where's the downside?

I don't see what killing marx would achieve.
All the dictatorships would find another populist ideology to dictate people with, and somebody else in the timeline will be critical of capitalism and become a symbol of its criticism.

Unless OP goes back in time to kill everything single one of them? and any rando being critical of capitalism gets his own time traveling assassin?

And as time goes on, it gets easier to criticize the ideology that sends time traveling assassin to any of its opponent.