Pasted from the Wikipedia article on Antihumanism to save me the time of typing out something substantially similar. Note particular the middle section on Marx.
"In social theory and philosophy, antihumanism (or anti-humanism) is a theory that is critical of traditional humanism and traditional ideas about humanity and the human condition. Central to antihumanism is the view that concepts of "human nature", "man", or "humanity", should be rejected as historically relative or metaphysical.
In the late 18th and 19th centuries, the philosophy of humanism was a cornerstone of the Enlightenment. From the belief in a universal moral core of humanity it followed that all persons are inherently free and equal. For liberal humanists such as Kant, the universal law of reason was a guide towards total emancipation from any kind of tyranny.
Criticism of humanism being over-idealistic swiftly began in the 19th Century. For Friedrich Nietzsche, humanism was nothing more than an empty figure of speech – a secular version of theism. Max Stirner expressed a similar position in his book The Ego and Its Own, published several decades before Nietzsche's work. Nietzsche argues in Genealogy of Morals that human rights exist as a means for the weak to constrain the strong; as such, they do not facilitate the emancipation of life, but instead deny it.
The young Karl Marx is sometimes considered a humanist, as opposed to the mature Marx who became more forceful in his criticism of human rights as idealist or utopian. Marx believed human rights were a product of the very dehumanisation they were intended to oppose. Given that capitalism forces individuals to behave in a profit-seeking manner, they are in constant conflict with one another, and are thus in need of rights to protect themselves. True emancipation, he asserted, could only come through the establishment of communism, which abolishes the private ownership of all means of production.
In the 20th century, the view of humans as rationally autonomous was challenged by Sigmund Freud, who believed humans to be largely driven by unconscious irrational desires.
Martin Heidegger viewed humanism as a metaphysical philosophy that ascribes to humanity a universal essence and privileges it above all other forms of existence. For Heidegger, humanism takes consciousness as the paradigm of philosophy, leading it to a subjectivism and idealism that must be avoided. Like Hegel before him, Heidegger rejected the Kantian notion of autonomy, pointing out that humans were social and historical beings, as well as Kant's notion of a constituting consciousness.[further explanation needed] Heidegger nevertheless retains links both to humanism and to existentialism despite his efforts to distance himself from both in the "Letter on Humanism" (1947)."