SHATILA AND SABRA, the refugee camps in West Beirut savaged over the weekend, are the latest obscure place names to be burned into our collective consciousness by the Middle East tragedy. Killers gunned down some scores if not hundreds of unarmed and apparently undefended children, women and old men, all of them apparently Palestinians who had survived weeks of Israeli fire on their miserable homes during the city's earlier siege. Pure vengeance seems to have been the single motive for the carnage, for there had been no complaint from any quarter that PLO guns were parked among these civilians. A more tragic demonstration of why a political solution must be found to the Palestinian refugee question could scarcely be imagined.

The prime responsibility must fall, of course, on those who did it. The gunmen appear to have been drawn from the ranks of the largely Christian militia of Maj. Saad Haddad, the renegade Lebanese officer set up as a friendly force by the Israelis some years ago, and the Christian Phalangist militia, also with Israeli ties, that was commanded, until his recent assassination, by Bashir Gemayel. At this point it is impossible to know whether the killing was done in specific retaliation for Mr. Gemayel's murder (some 26 others died in the explosion that took his life, by the way) or in a spree of generalized revenge and bloodletting or by way by advancing the political objective of removing the Palestinian refugee presence from sensitive areas of Lebanon. Nor is there much likelihood of a reliable future accounting, given the pervasiveness of mass murder in Lebanon's recent history and the lack of any system whatsoever for bringing the perpetrators of political violence to justice.

The Israelis, however, cannot avoid their own measure of responsibility. Their officials say now that the restraints placed on them by American diplomats and Moslem Lebanese politicians kept the Israeli army from imposing the firm presence in West Beirut that would have forestalled this tragedy. This is a lame evasion. The Israelis invaded West Beirut last Wednesday -- an operation that entailed shelling of the two camps where the massacre later was to take place -- precisely to fill what they insisted was a vacuum of authority created by the death of Bashir Gemayel. By their invasion they made themselves accountable for its consequences. Whether there was a degree of actual Israeli complicity in the events in Shatila and Sabra will no doubt be argued over for some time. Even if there was none, the Israelis still will not be absolved on the larger question. Notwithstanding their insistence that they were going into West Beirut to protect people, moreover, no physical signs had been reported of any trouble in that part of the city before the Israeli army entered. At the least, it appears, their entry created the conditions in which the massacre took place.

Mr. Reagan's statement of protest on Saturday was strong. It remains, true, however, that he and the rest of his administration -- and, for that matter, much of the American press -- were slow throughout the week to perceive the dangers of the Israeli slice into West Beirut. The administration found no public words at all on Wednesday, the first day. On Friday, even as the killings were beginning in Shatila and Sabra, Mr. Reagan at a political fund-raiser suggested that the Israeli operation had been prompted by a leftist militia attack -- none had been reported. He was still speaking at that time of the Israeli army's handing over its West Beirut positions to the Lebanese army.

In retrospect, another painful question arises. Where was the international peacekeeping force? The answer is known. The Americans, French and Italians sent in their men for the limited purpose of covering the evacuation of the PLO fighters from Beirut, and once that operation was completed, they pulled them out. Was that a mistake? Should the force have been left in place to protect civilians, especially Palestinian civilians, who had been left with no military protectors of their own? Should the Americans and the others have waited until the Lebanese army had actually shown its capacity to police the city? Were they so intent on minimizing their own casualties that they could not see the dangers to others that remained after their men departed? There would have been risks and possibly heavier costs if the peacekeepers had stayed on. There might also have been additional benefits. It is part of the somber picture that must be contemplated as the dead of Shatila and Sabra are mourned.