Marx's writings abound with moralistic condemnations. His villains are variously labeled human rubbish, hucksters, expropriators, and so on. Yet at root, Marx is an amoralist who denies the existence of any universal moral standards.
The selfish misconception that induces you to transform into eternal laws of nature and of reason, the social forms springing from your present mode of production and form of property - historical relations that rise and disappear in the progress of production - this misconception you share with every ruling class that has preceded you.
Manifesto of the Communist Party
The charges against Communism made from a religious, a philosophical,
and generally, from an ideological standpoint, are not deserving
of serious examination.
[T]he three classes of modern society, the feudal aristocracy, the bourgeoisie and the proletariat, each have a morality of their own... [W]e can only draw the one conclusion: that men, consciously or unconsciously, derive their ethical ideas in the last resort from the practical relations on which their class position is based... We therefore reject every attempt to impose on us any moral dogma whatsoever as an eternal, ultimate and forever immutable ethical law on the pretext that the moral world, too, has its permanent principles which stand above history and the differences between nations.
But does this position have any practical consequences? Indeed it does. Marx wants to do away with any moral barriers, any restraints of conscience, that might inhibit would-be revolutionaries. In particular, he wants to disparage any theory of "the so-called rights of man" (as Marx puts it) because total class warfare and human rights will inevitably conflict.
For Further Study
Carl Landauer, European Socialism: A History of Ideas and
Robert Tucker, ed., The Marx-Engels Reader
George Walsh, Marxism: Philosophy and
George Walsh,The Judeo-Christian Tradition