Prime Minister Menachem Begin, in the first hint of Israeli flexibility in seeking a settlement to the Lebanese crisis, said tonight he would allow encircled Palestinian guerrillas to leave Beirut by sea without giving up their personal weapons.

Begin's remark, in a parliamentary debate on his government's conduct of the Lebanese invasion, marked what could be a shift on Israel's previous insistence that all fighters from the Palestine Liberation Organization lay down their arms in total surrender and leave Lebanon or face an Israeli onslaught on West Beirut.

"We do not want to humiliate them," he said. "With their personal weapons, let them go in peace."

It was unclear whether Begin's offer--a short phrase in his 90-minute address--represented new Israeli terms in exchanges with U.S special envoy Philip C. Habib, who has been negotiating with PLO leaders through Lebanese political figures in Beirut. If so, it would soften the previous Israeli stance, which was described today by a high Israeli official as including insistence that the guerrillas surrender all their arms and leave Lebanon with only "their lives."

Allowing the guerrillas to retain personal weapons, presumably rifles and handguns but no heavy arms, would be a step in the direction of a reported PLO desire to avoid the total humiliation of surrendering all arms and moving defenseless out of Beirut. In addition, Begin reiterated previous Israeli willingness to allow the guerrillas to leave by sea rather than suffering the humiliation of passing overland through Israeli lines on the road to Syria.

"They are lowly murderers," Begin said, " . . . but they are human beings."

At the same time, Begin reiterated the Israeli threat to launch an all-out attack on West Beirut if the Israeli conditions for PLO surrender are not met, saying, "it would not be reasonable" for Israel to forswear this option against "the terrorists."

A high Israeli official charged earlier today that the PLO leadership was negotiating in bad faith and stalling for time in hopes of avoiding an Israeli assault without surrendering and leaving Lebanon. One sign of the alleged bad faith, he said, was a Palestinian attempt to retain some small arms to police refugee camps. He made no mention of a PLO demand to allow guerrillas leaving Lebanon to keep arms, but said the Palestinian tactics led Israel to view the situation with "great gravity."

His remarks seemed intended to increase pressure on U.S. and Lebanese negotiators who are dealing with PLO leaders in the encircled Lebanese capital to arrange the departure of Palestinian fighters.

In a carefully drafted Cabinet statement Sunday, Israel declared that all PLO members, without exception, must depart Lebanon. Backing its demand were overwhelming Israeli armored forces poised to attack West Beirut and the U.S.-supplied Israeli Air Force against which the guerrillas have almost no defense.

With this in mind, Defense Minister Ariel Sharon tonight told the Knesset, the Israeli parliament, that the guerrillas must "examine their consciences" over the Israeli demands, adding: "The government has given them a time limit to get out of their trap in Beirut."

Against this background, special U.S. envoy Habib was conveying a series of proposals relayed from PLO leader Yasser Arafat by Lebanese politicians and, through Habib and Washington, to the Israeli government in Jerusalem. It was these proposals and what the official called "the atmosphere of the negotiations" that prompted Israel's charges of bad-faith bargaining.

"We are under the impression that the PLO in talks with the Lebanese government and Ambassador Habib, that these talks are not going well, that the PLO is playing for time, that the PLO is lying, is trying to make the world believe they are ready to leave and so on, that they are playing for time," he said. "Therefore, we hope the United States and Lebanon understand that additional pressure will have to be brought to bear on the PLO."

The source, a high-ranking official with access to details of the negotiations, declined to say how the Israeli government could judge the "atmosphere" of the talks when its representatives are not participating. In theory, Israel is informed of PLO positions only through messages from Habib. The official indicated that Israeli officials are in touch with some Lebanese. In the past, Israel and Christian Phalange militia leaders have had close relations

As an example of bad faith negotiating, the official cited a PLO proposal that one of its units remain armed and attached to the Lebanese Army. This idea was rejected by the Lebanese government even before Israel passed on it, the official said.

The Palestinians' destination, the high official said, is up to whatever country is willing to receive them. The same official said last week the guerrillas would not be allowed to go to a country bordering Israel. But the Cabinet on Sunday discarded that condition