It's important to understand that 20th century communist states weren't just "communist" (there's no such ideology as communism); they were Marxist-Leninist, which despite the name is a rebranding of Bolshevism by Stalin. "Socialist" and "communist" are incredibly broad terms, and the idea that communist = implementing Marx's ideas. Now Marx's opinion would likely vary depending on time and place, but at least he'd probably condemn Stalin's USSR as an authoritarian hellhole. Beyond that I have no idea, but many Marxists who were contemporary to the things you describe condemned them and many others supported them, so we can't make a realistic guess without projecting our own values on him.

PS: I suspect you don't know much about Marx's ideas, so you should start from there. First, the dictatorship of the proletariat isn't necessarily an actual dictatorship (that's not how the term is used by Marx).