At its core, Rousseau's idea of human nature is an optimistic one. He sees human nature as intrinsically good when it gets away from social attachments and the corrupting influence of society. For Rousseau, human nature is a good impulse where the best of human beings are displayed. Rousseau's political theory is rooted in this idea of getting human beings into a social setting that is devoid of the corrupting elements of society. Rousseau's pivot towards a political and social order of amour propre over amour de soi is evidence of this.
The innate goodness of human nature that Rousseau sees as part of humanity is not something that Machiavelli shares. Machiavelli saw human nature as duplicitous. He was not suggesting that humans are evil, as much as they need to display different elements in different contexts in order to achieve what they want. There is a relativism that lies at the core of human nature, for everything is contingent on circumstance and condition. Human nature reflects this. For Machiavelli, politics was designed to engage in such manipulation. Morality and goodness were secondary to the political expedience of such elements. Rousseau believed that human nature was essential to the political experience to reflect intrinsic goodness. Machiavelli sees human nature as something contingent and conditional, able to show malleability in a political climate that nets the best result for those in the position of power.