Well, even like that I don't agree with your analysis of the Reign of Terror. It was a bad idea, but first you can't resume the French Revolution to it, and then, it was not that they "were really, really confident that things would be best if they could decide more or less ad hoc to kill tens of thousands". Robespierre, the head of state during the Terror, was against death penalty : http://www.marxists.org/history/france/revolution/robespierre/1791/death-penalty.htm . But he was against the death penalty in time of peace, not in time of war.

That's they key to understand it : the situation of French Republic was terrific. The League of Kings (that is, the rest of Europe) was in war against it, and there were many people inside France trying to destroy the Revolution from inside and go back to Old Regime. The Terror was a survival policy, much more than a policy of overconfidence.

If you look at the core texts of the French Revolution, like the Declaration of Human Rights of 1793 ( translation available at http://www.columbia.edu/~iw6/docs/dec1793.html ), which was inside the Constitution of the First Republic, they were really showing a huge distrust about themselves. Just a few quotes :

«
Art 9. The law ought to protect public and personal liberty against the oppression of those who govern.

At 12. Those who may incite, expedite, subscribe to, execute or cause to be executed arbitrary legal instruments are guilty and ought to be punished.

Art 28. A people has always the right to review, to reform, and to alter its constitution. One generation cannot subject to its law the future generations.

Art 31. The offenses of the representatives of the people and of its agents ought never to go unpunished. No one has the right to claim for himself more inviolability than other citizens.

Art 35. When the government violates the rights of the people, insurrection is for the people and for each portion of the people the most sacred of rights and the most indispensable of duties. »

Those clearly show a distrust into those who will wield power, including themselves. The French Revolution didn't fault by being overconfident in itself. The core difference was between the French Revolution and the American one was that the Americans weren't surrounded by hostile countries trying to crush them exactly at the same time in which they were facing civil war. The Montagnards applied the Terror because they were corned, not because they were arrogant.