THE INCREDULITY that met the Korean airliner tragedy sharpens the outrage and shock. Why? There are a hundred reasons why a Soviet fighter should not have shot down the strayed plane, and no good or explicable reason why it should have. Every consideration of common sense, military necessity, diplomatic discretion, humanitarianism and concern for Moscow's own globe-spanning air fleet would seem to have argued for a Soviet decision to deal carefully with an identifiable and unarmed plane flying at a familiar hour not far from a familiar route.

Could a single Soviet pilot or local commander somehow have gotten itchy, misread the wandering plane's situation, or acted without due consultation with political authority? It is noteworthy that the deadly sequence of pursuit and eventual kill unfolded over 21/2 hours, a period permitting ample consultation and one falling during a wide-awake evening timespan in Moscow. It cannot be ruled out that this brutal affair was no more or less than a product of the normal working of the Soviet system. There is bound to be speculation, however, that it resulted from some devious Kremlin power play or from an intent to make a cautionary display of coldbloodedness.

No conclusive answer is likely to come soon, if at all. What can be said now, however, is that the official Soviet answer delivered yesterday is evasive and inadequate in every respect. It ignored the Soviet responsibility for the downing of a plane carrying 269 passengers (including an American congressman, Lawrence McDonald of Georgia, and some 30 other Americans) and crew. It ducked the central questions posed by Soviet conduct throughout the incident. It altogether failed to address the situation's requirement for compassion for the dead.

The savage act caught the Reagan administration at a moment when it has been seeking some practical advantages in dealing with Moscow without unduly offending the president's traditional constitutents and beliefs pointing in the opposite direction. Weighing these diverse considerations, the administration pronounced itself appalled but, while demanding that the Kremlin provide a full accounting, it made sure the broader negotiating lines stayed open. This seems to us a reasonable reaction. It is a time to absorb, with as many facts as can be gathered, the meaning of a shocking event.