The difference between Rousseau's and Machiavelli's views on human nature is stark. Rousseau believes that mankind is essentially good. In fact, this view of man is usually contrasted with Thomas Hobbes, who asserted that mankind was self-serving and depraved, driven to take from others in pursuit of self-preservation. Speaking of man in his natural state, Rousseau observes:

Such is the pure emotion of nature, prior to all kinds of reflection! Such is the force of natural compassion, which the greatest depravity of morals has as yet hardly been able to destroy! 

Rousseau argues that it is the trappings of so-called civilization that have corrupted man, and created a need for government. Men, in short, were innately good and decent, and were corrupted by their entrance into society.

Rousseau is a idealist and a theorist, but Machiavelli, at least in writing The Prince, is a hard-nosed pragmatist. He is less interested in how man became corrupted than in showing that rulers ought to deal with men as they are, not as they were or should be. His view of human nature is accordingly pretty bleak: 

Because this is to be asserted in general of men, that they are ungrateful, fickle, false, cowardly, covetous, and as long as you succeed they are yours entirely; they will offer you their blood, property, life and children, as is said above, when the need is far distant; but when it approaches they turn against you. 

Machiavelli emphasized the need for a prince to be every bit as duplicitous as his subjects in order to rule them. They had to behave as tyrants, if necessary. Rousseau, on the other hand, posited that the best government would be one that ruled based on the "general will" of the people. So their very different views on human nature deeply informed their views of how people must be governed.