What seems lost on many people about the dangers of collectivism is the inter-connection of ideas, political systems, and morality.
Let us compare the predominate ideal-forms of modern polities: Communism and capitalism. Fascism is not an ideal-form because it is a mixed form of capitalism and socialism; one might say it is the corruption of capitalism by socialism; fascism obviates private property through coercive control of the economy. Socialism is parasitical on capitalism and is not predicated on the creation of goods, only on the redistribution of them.
Socialism is merely a watered-down form of communism and, if it is not eliminated, it will drown a capitalist economy. Socialism is like the crack in the edifice of the dam against communism; once it begins to appear, one must patch it at the philosophical source or it will flood the society with ever-increasing demands on the life and labor of productive individuals. This will proceed until it becomes a moral imperative for individuals to cease creating and producing to subsidize the inherently inhumane and oppressive regime.
Capitalism reinforces an appreciation of the individual by necessity through its legal-rational structure. Private property is intended to deter greedy citizens and power-hungry governments from forcing their will onto individuals in the economy. This leads to the formation of a "civil society," since the legal arrangement of human beings provokes at a minumum what Immanuel Kant referred to as "unsocial sociability" and if capitalism is combined with Jeffersonian local self-governance, the spontaneous cooperation of human beings to solve "collective action problems."
Collectivism, alternatively, necessarily centralizes decision-making in a small body of oligarchs. This is an arrangement ripe for the abuse of all under the body's control, due to man's inherent nature as a self-interested being.
Capitalism in its ideal-form does not lead to mass murder; this is especially true if Jeffersonian republican political forms prevail. In a capitalist system with assumptions of private property, execution of a person is the murder of an actual human being. A person ultimately owns himself.
Under communism, since one does not love individuals, only humanity, the execution of a troublesome person is a small "sacrifice" for the benefit of the common good. After all, one cannot execute humanity.
Under socialism, the same perverse logic applies on a milder scale, and it is crucial to recognize the same ultimate conclusion as communism on the moral continuum. Under socialism, the government doesn't line people against the wall, they withhold medical care until the burdernsome old people expire. Again, small sacrifice for the "common good."
This is why socialism, fascism, communism, all forms of collectivism, are extremely dangerous ideologies. Human morality is wont to degrade in the absence of legal-rational reinforcement. Pragmatism, as a philosophy, may be one of the most dangerous "isms" of all. This is because pragmatism subsumes morality by posing social and political questions in the form of cost-benefit analysis; this is often a psychological necessity when chalking up the cold calculations of acceptable "sacrifices" for the "common good" in altruistic collectivist systems.
People tend not to see themselves as evil as they are committing great atrocities. They see themselves as well-intentioned or motivated by expediency, dire circumstances, or grave danger. It takes rationalization to attenuate the guilt that ensues from the violation of another human being's life. Pragmatism, as applied to the coercive manipulation of human beings, provides such rationalization. Pragmatism holds the door open wide for collectivist ideologies.
That is why Ayn Rand's Objectivism might be said to oppose pragmatism writ large; pragmatism formulates a misunderstanding of long-term causes and effects and the interrelationship of ideas, reality, and morality in society. Objectivism reinforces life, liberty, and property with a consistent philosophy resistant to collectivist corruption.