Did Rousseau and John Locke agree on assumptions about human nature or did Rousseau embrace some of the assumptions of Locke? Their disagreements come in views about what constitutes "human freedom" and man in the state of nature. If man is born free but everywhere he is in chains, then it is institutions that enslave him, and the issue is how to free man from such enslavement, as far as Rousseau is concerned. Locke had the answer, the Liberal (parliamentary formula) based on property, because man is governed by reason and tolerance. But what if he is a slave? In any event, Rousseau's influence comes more from a 16th-century "Geneva Calvinist" background than from 17th-century Dutch-English influences.
As far as the idea that the environment shapes human nature, and that if one wishes to shape society one must go to the roots of the societal dynamics, this is indeed a very "Age of Reason" concept that I see as rationalist in its intellectual scope and social democratic in its political intent. Never defining the "general will," we must define it as we assume he would have it, and I am assuming it was the will of the collective body that entered into a social contract. This too is more Calvinist than Lockean, because in Locke's world the propertied classes were the shareholders of government and government more or less like the management of a company (very Dutch model).
By arguing that people must be forced to be free, one can conclude that Rousseau must have been "totalitarian," but why reach such a conclusion and not put Rousseau in his contemporary setting? I see the issue as a philosophical disagreement with Locke on individualism v. general will during the Age of Absolutism. Let us accept that Rousseau influenced Robespierre, as indeed it the case; and let us accept that Robespierre was responsible for the deaths of thousands as he was. Is it not true that even monarchists agreed after his execution that the ultimate goal was not to clean out the public treasury and use public office for personal gain, but to create a better society, one based on greater justice for more people that were actually driving the fanaticism of the Revolution and the provisional government was swept away by mass pressure?
One could very well argue, in the end 30,000 (approximately) killed are 30,000 dead owing to political executions, and who cares about intent to build a better society when your father's head has just rolled into the basket? No argument from me there. But do we not at least acknowledge the ultimate goal and ideological makeup to understand Robespierre the Incorruptible and the fanatically idealistic Jacobins? (I hope that this does not sound like an apology for them, but point of clarification.) Were the Jacobins Nazis or is this another case of lumping together everyone who differs from Liberalism!
There is a great deal on this thesis, but it simply lacks substance and it is a huge stretch. The thesis that Jacobins are precursors to Nazis is one of the early Cold War, when mostly US scholars tried to "bunch up" Fascism, Nazism, and Communism for purely political reasons. Is there intellectual integrity behind such arguments, or are they more political? From the Cold War to the war on terrorism there have been many efforts to de-legitimize progressive philosophers in any manner possible. This is the case with Rousseau that some have endeavored to stigmatize as another 'Liberal Fascist' thinker.
By so branding philosophers like Rousseau, neo-conservative apologists argue that Fascist movements are in essence 'left-wing'. If you want to blame anyone for Mussolini and Hitler, blame Rousseau and other such progressive philosophers. The ultimate goal, is to argue that any progressive ideology is an anathema, and that people ought to accept conformity to the neo-liberal status quo which in many respects entails economic authoritarianism.