Both of these men, to say the least, held views of human nature that were complex, but they can be said to have one thing in common: they both argued that society, as currently arranged, tended to inhibit, if not destroy, the fulfillment of man's nature. Marx viewed human nature as essentially social. One clear statement of this concept can be found in his essay "On the Jewish Question," in which he argued that political rights and human rights were not the same thing. He described man as a "species-being" that was unnaturally alienated from others by the very concept of political rights. Man's urge was fundamentally social. This concept was underscored in his later writings, especially Das Kapital, where he argued that the economic forces in society, i.e. capitalism, tended to alienate men from each other by making them into mere economic agents in competition with others. He expanded this thesis by pointing out that a capitalist system also alienated people from the value of their labor, both socially and economically.
Nietzsche thought of human nature in terms of the need each person has for actualization, for the fulfillment of natural urges. The word he often used to describe these urges was "will," and the "will to power" was an especially powerful aspect of human nature. The nature of this "will" has been debated by Neitzsche scholars, but it was fundamental to life, the ultimate human drive. Where Marx saw man as ultimately social in nature, Nietzsche viewed human nature as radically individual, and his writing, though aphoristic and often contradictory, tended to emphasize struggle among people. But like Marx, he thought that basic institutions and societal forces alienated people from their authentic nature. Moral codes, particularly those imposed by Christianity, struck Nietzsche as especially repressive, keeping men from fulfilling their deepest urges. A focus on the ways these thinkers both believed modern man was restrained from living in a natural way, that is according to human nature, would be a good starting point for an essay.