The point of Marxist communism was that a "violent revolution" (his words) and then "dictatorship of proletariat" are absolutely required for cleansing and "rebirth" of society.
Social democrats argued that this "violent revolution" may bring just as much evil as good and thus opted for a peaceful change. Marx accused them of being "reactionary".
The history of Eastern vs Western Europe clearly demonstrates that Marx was wrong and s-d was right.