STORIES OF THE murders of prominent people carry a shock of their own, in part because prominent people are assumed to be immune from murder, at least from murder with any passionless motive like robbery - the way Charles Frankel was murdered last week. Mr. Frankel and his wife were shot to death by robbers in their home in a wooded area of Bedford Hills, N.Y. Either before or afterwards their killers also murdered two other people in a nearby home, from which they stole a safe. "Why would a professional burglar kill four people?" asked Acting District Attorney Thomas A. Facelle, posing the immediate, practical question. The wider question is simply: Why do such things happen?, which is the kind of question you would have asked Mr. Frankel.

Mr. Frankel's abiding intellectual interest was the problem of liberty. As professor of philosophy and public affairs at Columbia; as assistant secretary of state for educational and cultural affairs under Lyndon Johnson; as author of a dozen books and who-knows-how many articles, he saw the idea of freedom as the central condition of civilization. The freedom of criticism , of course. The freedom of dissent, naturally. Yet the freedom to be let alone, as well: "The function it serves, the long-term social interest it represents, is a complex of basic goods: family, free friendship, love and personal communion, solitude and self-confrontation, religious or philosophical integrity, the intimacies and intensities that are only possible for rational people when they can choose their company and keep the prying world away."

It is not an idea often heard these days, that the counterbalance to self-expression is the private development of integrity. Behind it is nothing selfish at all, but rather the firm belief that society is served by privacy. Mr. Frankel saw freedom as a social, not just a personal good. In his writings he commonly used words such as "honor," "nobility," "grace," "elegance," "truth," and "reason" above all. He loved an intellectual battle mainly because he trusted his adversaries to be reasonable. His first book was "The Face of Reason." Later came "The Case for Modern Man," in whom he also had faith. All of which showed plainly in his own face - big, honest, alive, almost always with a cigar in it.

What Mr. Frankel would have made philosophically of his murder and that of his wife, and of the two other innocents is not hard to guess. He would have seen the acts as aberrations, not at all affecting his fundamental optimism, or trust in rational life. "Our disappointments are real," he wrote in "The Case for Modern Man," "but they are real because our powers are great and our expectations legitimately bright," That may be so. But there are moments such as this when we have a right to dwell more on the disappointments than our great expectations, and to weep for the difference.