AGAIN PALESTINIANS are dying in Sabra and Shatila. True, it is not September 1982, when Israel's Lebanese Phalangist allies, taking revenge for the killing of their chief, murdered hundreds of civilians whose PLO protectors had been removed from Beirut on the promise -- by the United States and Israel, among others -- that they would be safe. This time Lebanese Shiite militiamen of Amal are seeking to prevent the slightest rebirth of the "state within a state" that the PLO set up in Lebanon in the 1970s. But, though there is nothing like the earlier hue and cry, Palestinians are dying in Sabra and Shatila all the same.

The Shiite community is the largest in Lebanon, one of the most socially aggrieved, and the one with the most to lose from a PLO rebirth. Such a development would diminish the Shia community's national place, draw punishing Israeli reprisals, feed its fundamentalist wing and invite further Syrian intrusion. Amal is, in the Lebanese fashion, unforgivably bloody-handed: accounts of its shooting of PLO hospital patients are not softened by accounts of the PLO's shooting of Shiite patients. But the Shiites are not without a political logic of their own.

That leaves, of course, the Palestinians: It leaves them exposed in the Palestinian slums of West Beirut and no less vulnerable elsewhere in southern Lebanon. It is quite possible that the PLO itself, either the Yasser Arafat mainstream associated with Jordan or the Abu Musa group manipulated by Syria, provoked this latest upheaval. But that does not alter the basic fact that the Palestinian people are still rattling around the Middle East, miserable themselves and making life miserable for others.

Enter King Hussein. Eventually the Syrians will have to be brought back into Arab-Israeli diplomacy, but the current minority Alawite regime in Damascus is more immediately concerned with its own survival. The Jordanians are the necessary and only available expediters on the Arab side. King Hussein has been in Washington, playing an uncharacteristically bold and helpful role.

The king now claims PLO backing for the idea of taking a joint Jordanian-PLO delegation, under the "umbrella" of an international conference including the Russians, into direct peace talks with Israel. These talks would be based on Security Council resolutions 242 and 338, which embody the broad concepts of a settlement, and would be aimed on the Arab side at creating a Palestinian West Bank- Gaza state in confederation with Jordan.

Much diplomatic work remains to be done, not least by the United States, to put this proposal on a firm footing and to match it up with Israeli proposals. In particular, the PLO must come forward to state explicitly its own policy. Presumably few people need to be reminded of the times that positive-sounding arrangements between Hussein and the PLO have crumbled.

But some of the elements stated by the king -- acceptance of 242 and 338, direct talks -- are, if validated, of tremendous potential value to Israel. Other elements -- a seat at a table, a state in confederation with Jordan -- are of no smaller potential value to the Palestinians, whose alternative is, after all, Sabra and Shatila. The promise now visible is merely a glow on the far horizon and it remains to be made real, but for that prospect the man responsible is King Hussein.