TO THE PICTURE now being assembled of the West Beirut massacre must be added the chilling and politically explosive story of the private "Christian" army known as the Lebanese Forces. The Post's Loren Jenkins reports the operation was planned by Bashir Gemayel, the Forces' commander and president-elect of Lebanon until his assassination on Sept. 14, and his confidantes. Though the plan did not call for a slaughter, Mr. Jenkins writes, the intent was not simply to sweep up armed Palestinians but also to terrorize the camps' occupants by way of forcing Lebanon's half- million Palestinian refugees to flee the country.

This account is not the final one, but it will have a considerable ring to Lebanese familiar with 1) their country's indulgence of fratricide over the years and 2) the animus shown toward the Palestinians and especially toward the PLO, at least in the past, by Bashir Gemayel and his militia and his Phalangist party.

Even before the massacre, the earlier bloodletting had cast a long shadow over Lebanon's quest for conciliation. Afterward, Lebanon's political community seemed determined to look past it in the name of healing: confronting it was too hard. But now the horror of the event and the attention being focused on it by the international press and by Israel's inquiry into its own role are compelling Lebanon to face it head-on.

More facts are bound to come out, though perhaps not from the investigation being conducted by the Lebanese Forces' chief of intelligence -- the very man reported to have commanded the operation. What does come out will hardly be calculated to ease distrust among Lebanese. Nor will it likely comfort President Amin Gemayel, Bashir's brother and political heir. He is damned one way if he knew what was going on -- he denies he did -- and another way if he did not. He will be pressed to bring the Lebanese Forces and the other militias under a central Lebanese authority that does not yet exist.

Israel and Lebanon are very different kinds of societies. All the same, elements in both Israel and Lebanon do share the basic attitude that perhaps more than any act of either commission or omission led to the massacre: a dehumanizing perception of a third people, the Palestinians. The Israeli authorities wanted PLO guerrillas ostensibly hiding in the camps to be, in their parlance, "cleaned out." The whole purpose of their invading Lebanon, after all, had been to eliminate the Palestinians as a fighting force and a political presence. The Lebanese Forces needed no urging to enter the camps. They acted for sectors in Lebanon holding, with their Israeli partners, that Palestinians are an alien people deserving neither respect nor rights.

In their fashion, the Isralis are now confronting their responsibility in the massacre and, necessarily, in the events and policies leading up to it. The Lebanese cannot avoid a parallel pursuit, in their fashion, painful as it will be.