IT BEGINS to look as if Chilean President Augusto Pinochet may have ordered or at least countenanced the assassination of his most telling critic, Orlando Letelier, in Washington 21 months ago; a colleague of the former ambassador also died in the plot. And if this is true, there is, then, not only justice but a sweet irony in the possibility that the investigation into Mr. Letelier's murder may lead to the end of President Pinochet's stay in power.

Here's what apparently happened after federal investigators found indications that an American expatriate named Michael Townley and a Chilean had come to the United States and met with Cuban exiles a month before the assassination: The State Department quietly but firmly insisted that the Chileans cough up Mr. Townley.The Chileans did. Mr. Townley made the best deal he could for himself and started cooperating with prosecutors. The result is that murder charges are expected to be filed here against a number of former Chilean secret police officials, including the former No. 1 man, a close confidant of President Pinochet.

It is no surprise that, in Chile, dismay and anger over the investigation are yielding in part to the feeling that the Pinochet government has outlived its usefulness. It is one thing to act against "communist" forces within Chile; it is quite another to reach out to kill a critic in the capital of the very country on which Chile is most dependent - and to be found out - and to have the very man held responsible for the assassination remain president.

As it happens, a certain measure of normalization has been returning to Chile. Political arrests have diminished, and political prisoners have been released, though there has been no accounting for the hundreds who "disappeared" in police custody.The people at the bottom have not benefited, but the economy has otherwise advanced. Fissures within the junta have produced limited, semi-open politics, and the leash on the press has loosened.

We note those developments not to congratulate the junta but to make the point that President Pinochet no longer has a reasonable pretext, if he ever did, for thwarting Chile's return to constitutional government. That the Letelier assassination is now being laid on his doorstep provides additional incentive for him to step down. He would, presumably, come under the protection of the amnesty decree by which he has attempted to purchase the continued loyalty of the various criminals in his regime.

Chile, after all, has not only a recent but a rich tradition of constitutional government, one going back well into the 19th century. In the Christian Democratic Party, moreover, it has a respected and substantially intact political organization that, with other political elements, could assume the transitional task of steering Chile back to its democratic traditions. In short, although the continuation in office of President Pinochet means national disgrace and continuing international isolation, the alternative to him is not chaos. Does he wish to be remembered, in some measure, as a patriot? Then he should resign.