NOTHING CAN TURN out well in the matter of Billy Viscidi, but by now things might have turned out a good deal worse were it not for the tact and judgment shown by the Fairfax police and prosecutors. By keeping a lid on the case - by not talking indiscriminately to reporters or venturing guesses as they occurred - the Fairfax officials have not only prevented a public panic, they have also done their best to protect an already beleaguered family from public pryings. The latter especially has been no easy feat, given the air of speculation in which the Viscidi family now lives.

At first the case seemed terrifyingly common - a child was missing. As soon as that is announced, all the worst fears of a community come alive. A maniac is on the loose. Just as quickly, the authorities try to catch the kidnapper and put the community at case. From July 25, when Billy Viscidi disappeared, to Aug. 12, when his body was discovered in his family's backyard, that is exactly what the Fairfax officials did. They effectively discharged their responsibilities toward the Viscidis and the public, both of whom had an urgent interest in finding Billy and his alleged abductor.

After Aug. 12, however, the temper of the case turned inside out. A blood stain was found in the living room carpet. Billy's brothers weren't talking. Suddenly it was clear that Billy had not been abducted and, therefore, that there was no maniac on the loose. What had been a public concern was now a matter of public curiosity, and the same public that had shared the terror of the Viscidis now peered in at the Viscidis with a different kind of terror, sensing - as it has only been reasonable to sense - that whatever happened to Billy happened within his home. At that moment the responsibility of the Fairfax officials shifted from protecting a community to protecting a case, and the feelings of the Viscidis as well.

The latter should not be underestimated. To be sure, by remaining discreet, the police and prosecutors have kept their legal case - if there should be one - unimpaired. But they have also shown a keen sense of propriety in this business. If - as it is assumed - we are dealing with a family tragedy here, the officials have not allowed it to be confused with a family scandal. They have behaved with admirable discretion and restraint.