The root conviction which underlies this [that creating the ideal society is worth killing people] is that the central questions of human life, individual or social, have one true answer which can be discovered. It can and must be implemented, and those who have found it are the leaders whose word is law. The idea that to all genuine questions there can be only one true answer is a very old philosophical notion. The great Athenian philosophers, Jews and Christians, the thinkers of the Renaissance and the Paris of Louis XIV, the French radical reformers of the eighteenth century, the revolutionaries of the nineteenth—however much they differed about what the answer was or how to discover it (and bloody wars were fought over this)—were all convinced that they knew the answer, and that only human vice and stupidity could obstruct its realization.We see this in any society in which some external Law develops against which one must compare oneself. Individually, this results in the development of guilt (Medieval Christian Europe, Islam, the Roman Republic/Empire under Roman Law). Socially, this can and too often does lead to atrocities designed to protect the Law (the Inquisition, Islamic terrorism, crucifying rebels and campaigns against philosophers and Christians).
But in collective guilt cultures, the scale of the atrocities can increase exponentially, because the scale of actions condemned is much greater. In guilt cultures, so long as you abide by the rules of the external Law, you can pretty much do whatever else you want to do. That Law actually covered and covers far less than one might realize. In fact, anyone could come under the law, so in many ways it was more inclusive than is the Law that creates collective guilt. After all, one is guilty for being rich from market activities (in the case of Marxists), the member of a particular ethnic group (the Jews in the case of national socialist Germany), etc. We can perhaps count ourselves lucky that as collective guilt culture emerged in the capitalist West in the past several decades that it has had so many "guilty" -- men, those of European descent, the rich (but only if the rich own private businesses), etc. -- that it becomes increasingly difficult to commit mass murder against them. But that still does not mean that pernicious ideas aren't behind the Law underlying collective guilt culture.
As Berlin points out, the Law of guilt and collective guilt cultures are incompatible with people pursuing their own goals: "The central values by which most men have lived, in a great many lands at a great many times—these values, almost if not entirely universal, are not always harmonious with each other." The discoveries made in the pursuit of knowledge makes some people uncomfortable. "Creative imagination and spontaneity, splendid in themselves, cannot be fully reconciled with the need for planning, organization, careful and responsible calculation." New things disrupt perfection.
But most importantly, the Law of both guilt and collective guilt cultures cannot tolerate mercy, for mercy means allowing those who are harming society to "get away with it." If one's view of justice requires society to match some ideal, mercy allows imperfection in. We cannot allow imperfection in, thus justice turns unmerciful. When that happens, the only solution is to introduce stricter and stricter laws and punishments. Even three strikes and you're out can become too lenient. Perfect organization of society cannot tolerate difference.
But self-organizing network processes not only can, but are most complex and robust when heterogeneous. But such processes are, fundamentally, anarchic. Rules emerge naturally through the interactions people have with others, so the world is certainly knowable and (humanly) predictable, even while also being fundamentally uncertain. But that just makes it a more natural process. Utopia is truly and always nowhere. Attempts to create it will always result in that utopia being built on a foundation of corpses.