FANATICS HAVE their dreams," said Keats, "wherewith they weave a paradise for a sect." The Rev. Jim Jones wove a "paradise" for his sect, the People's Temple, in the jungle of Guyana; and now out of his crazy zeal has come a horror that, if not predictable, at least followed its own grisly logic: five Americans murdered by Mr. Jone's followers, and at least 385 dead followers themselves, including Mr. Jones - some murdered, some the victims of apparent suicide. One says "apparent" because the Temple population's well-rehearsed ritual of taking poison was accompanied by semi-automatic gunfire. Just how many of the 385 willingly saw what Mr. Jones had advertised as "the dignity of death, the beauty of dying," and how many were forced, remains unknown.

In the sheer revulsion of the moment, the question is the old one: How do such thing happen? We go along dreamily on a bright fall weekend, when suddenly, from a place no one can locate on a map, comes news, once again, of the human mind in some of its feeblest and ugliest and most terrifying manifestations. And surely there are in our contemporary and recent experience models and analogies - political and occult - for the hypnotic frenzy, the mesmerization by the leader, the glorification of bloodthirsty acts. We all know that it isn't always easy or even possible to tell a demagogue from a saint. That much we can grasp. But the darker level of devotion at which one is ready to forfeit his life for the ravings of a fanatic - that, most of us will never know or understand. There is a depth of helplessness and surrender at which all the dignity and beauty of the mind cease to matter. That is the place of which Rev. Jones dreamt, and that he found in Guyana.

Unlike the inaccessibility of the acts of Rev. Jones and his followers to the ordinary mind, the courage of Rep. Leo J. Ryan (D-Calif.) and the newsmen and others in his party is readily recognizable. It was Mr. Ryan's desire for first-hand information that had taken his party to that remote and dangerous place. And that was characteristic of Mr. Ryan; time and again over the years, he had shown a penchant for going to the scene - a week in Folsom Prison to learn about conditions there, a trek to Newfoundland to publicize the brutality of hunting harp seal pups.

What distinguished this trip from those others was the degree of danger and irrationality that he expected he might find. Mr. Ryan, by all accounts, was well aware of the violent strains in the People's Temple cult. He had begun looking into the sect's activities because friends and constituents who were former members or who had relatives caught up in the group had told him of its cruelties and intimidation of questioners. Apparently the more he learned, the more dogged his inquiries became. Despite warnings that going to Guyana might be perilous, he chose to take the risk.

The same may be said for the three newsmen NBC reporter Don Harris, NBC cameraman Robert Brown and San Francisco Examiner photographer Greg Robinson - who were slain in the ambush. They too were there on business, along with other journalists who also certainly knew the group's habits of pressuring journalists. Yet they too pursued the story - in Mr. Brown's case, by filming the airstrip attack with breathtaking courage until the moment he died.

Like Leo Ryan's bravery and persistence in the pursuit of his duties, those same virtues on the part of the newsmen represent the single feature of the nightmare events that can be contemplated with understanding, and admiration.