Overview
I'm going to trace a theme — the relation between individual human nature and society — through all the authors we have read.
1. Plato subordinates the individual to society (cog in a machine). He isn't concerned with values like individual liberty. In one sense, then, he's definitely not an individualist. However, his picture of individual human nature (the three-part organization in the soul) doesn't owe anything to the social context. The idea seems to be that human individuals are like that anyway, independently of society. The point of living together in societies is to provide more efficiently for individual goals (food, clothing, shelter). There's no hint of the idea that human individuals are what they are partly because of living together in societies.
2. In Hobbes the idea of society as external to human individuals (not part of what makes them what they are) is explicit (Plato doesn't consider such questions head-on). For Hobbes, individual human nature is constituted by a bundle of appetites and aversions, including notably an appetite for power (a higher-order appetite; power is the ability to secure the objects of one's first-level appetites). Left without restraint, individual human natures would clash, in a war of each against all whose effect would be that no one did well. Society is an artificial construction to avert that disastrous outcome. The artificial construction exploits elements in individual human nature so as to make anti-social behavior unattractive. On this view, then, society is an artificial frame within which an independently constituted individual human nature operates.
3. In Rousseau, human nature (in a sense that works with a contrast between "natural" and "artificial") has pretty much disappeared from the scene with the development of social organization; human beings have become unnatural beings. (This connects with Rousseau's depiction of modern life as full of hypocrisy.) Contrast Hobbes' picture, in which the effect of social organization is just to change the incentives and disincentives that confront individual human nature, which is still present and operative. In another sense, "human nature" means "what human beings are like" (at a time), and using the phrase this way we can put Rousseau's thought by saying human nature has changed (for the worse, he thinks). Either way, we can say human nature has a history: either it stops being operative, or it changes.
4. Marx has the idea we find in Rousseau, that human nature, as what human beings are like (at a time), changes with developments in social (in particular economic) organization. He also has a conception of human nature (at any rate we can put it in those terms) as a potential, something human beings could and ideally would live up to — ideally, human beings wouldn't regard social ties as external to their true being. This enables criticism of existing arrangements, on the ground that they prevent human beings from living a properly human life.
5. Freud reverts to an echo of Plato's three-part soul. But Freud's version has a distinctively post-Rousseau (post-Marx) twist. Whereas in Plato it looks as if human individuals have that internal structure anyway, independently of their living in societies, Freud connects the development of the super-ego very closely to socialization. (That's why he thinks the sense of guilt is a problem specifically of civilization, not a problem of individual human nature independently of civilization.)