DEFENSE MINISTER Sharon's statement in the Israeli parliament clarifies the key aspect of the West Beirut massacre. He acknowledged that the Israeli army had not only planned the entry of Lebanese Phalangist militiamen into the two Palestinian neighborhoods, but had requested it--so that PLO guerrillas asserted to be in hiding could be "cleaned out" without Israel's taking casualties.

So there you have an official admission of what was at the least the staggering error of judgment that produced the tragedy -- during an agreed cease-fire, no less. "You don't have to be a political genius or a decorated general," opposition leader Shimon Peres said to Mr. Sharon, "it's enough to be a village policeman to understand ahead of time that these militias -- in the wake of the murder of their leader (Bashir Gemayel) -- were more liable than ever to sow destruction, even among innocent people."

Mr. Sharon added that Israeli officers had urged their Phalangist contacts to spare civilians, that he had not dreamed a massacre was possible and that his command acted to halt the carnage as soon as it learned of it. These and similar statements tending to absolve his government created a storm of revulsion and disbelief in the Knesset, and for good reason. First of all, the statements are hard to believe or beside the point. Second, their thrust is contradicted by the emerging specific accounts of what Israelis knew, and of which Israelis knew it, and of when they got their information.

It is now undeniable that the Begin government by commission and omission made this massacre possible. It then stonewalled and raised diversions ("blood libel") for four days. When it did go public, it did so in a way leaving heavy questions, not least among the Israeli public, about its candor and its competence alike. The resultant political implications are, as the Reagan administration wisely believes, best left to the Israelis themselves to work out.

There are some who observe, by way of being fair to Israel, that Israelis did not pull the trigger and that, furthermore, the nominal leader of those who did pull the trigger has just been elected president of Lebanon without anyone's so much as raising the topic. The observation illuminates an interesting set of differences between Lebanon and Israel.

The Lebanese may be unable to address the question of responsibility, but Israel is throwing itself into it in a way that defines its essence. Mr. Begin's rejection of an independent inquiry has not slaked the demand of broad sectors of the public, including members of his own government and party, for a full accounting. There is a double standard by which Israel is judged, and it is turning out to be Israel's pride.