Those who can make you believe absurdities can make you commit atrocities.
Like Hobbes, Bossuet, and Locke before him, Voltaire understood that theories of government are based on prior understandings of human nature. If your view of human nature is accurate, you have at least a chance of forming a rational political theory. But if your understanding of what it is to be a human being is incorrect, then your view of the relationship between society and government will be not only irrational, but also dangerous.
One need only think of names like Josef Stalin, Vladimir Lenin, Mao, Hitler, Pol Pot, and Daniel Ortega to understand these dangers.
But seemingly innocuous and even well-intentioned misunderstandings lie behind these epic brutalities. Kindhearted and benign philosophers, speculating about how to form ideal societies, can unintentionally fuel barbaric dictatorships. It was this about which Voltaire was warning us.
Our first example comes from Carl Rogers, a writer who has made excellent contributions to the fields of psychology and counseling. There is no doubt that his writings have benefited progress in these fields. But Rogers also committed at least one major gaffe, when he wrote that
Experience leads me to believe that it is cultural influences which are the major factor in our evil behaviors.
Likewise, Abraham Maslow discovered precise insights into human thought, and is a scholar of major stature. Yet he too made the occasional blunder:
Sick people are made by a sick culture; healthy people are made possible by a healthy culture.
The bottom line impact of these texts is to invite would-be revolutionaries to re-design, not governments, but societies, in the hopes of re-casting human nature itself.
If cultures were the source of evil, then it would be the noble duty of philosophers to re-wire societies. If this were true, then by re-configuring our cultures, we would be able to rid ourselves of evil. These views firstly eliminate personal responsibility, and secondly demand social revolutions until that arrangement of culture is found which does not nudge humans toward evil.
When Voltaire issued his warning, he perhaps had at least one specific individual and one specific situation in mind: the philosophy of Rousseau and the French Revolution. Rousseau's views can be summarized briefly as the assertions that humans are born essentially good, that society makes them evil, that humans and society are perfectible, and that society, not the individual criminal, bears the blame for evil. Voltaire was acquainted with Rousseau's views; he was not acquainted with the French Revolution, which occurred after his death, but Voltaire and others could foresee the general trend which produced the large-scale atrocities which would constitute the revolution.
Rousseau's thought, and the revolution which it fueled, were based on the premise that it was society, not government, which needed to be changed. This premise leads to the conclusion that anyone who functions in society (which is pretty much everybody) must follow the directives given by those who are in the process of re-designing society. Absolute obedience is needed; the social engineers need everyone to play his part according to instructions. If a person fails to take his place in the new order, it is morally justifiable (to the social engineer) to get rid of that person in the most expedient manner: which is why thousands of innocent civilians, including women and children, were executed in the French Revolution. The people must patiently follow instructions, because in the course of re-designing society, we'll have to keep tweaking and adjusting until we get it just right.
Of course, as we see in the French Revolution, we never do get it "just right" - as the revolutionary leaders of France kept issuing new and different plans every few months, often in contradictory fashion, to adjust French society until they had it just right. In attempting to get to this imagined society, more and more areas of human life must come under the control of the social engineers (most of whom probably honestly did believe that they were going to do something really beneficial for people), and so control of the press, religion, and speech were handed over to the revolutionaries, so that they could adjust more precisely the details of society: and so the revolution which was begun in order to create more freedom ended up taking away more freedom.
To contrast the French Revolution of 1789 with the American Revolution of 1776, we can say that the Americans left their society largely intact, and merely changed their government. The result was that the French, who began with radically large claims to freedom, ended up with less freedom; while the Americans, who began with more modest claims to freedom, ended up with more freedom.