IT IS SAID that the Israeli gunman who shot up the Dome of the Rock Mosque, killing two Arabs and precipitating bloody riots, was a deranged individual, the implication being that there will always be crackpots to deal with but they should not be given a disproportionate importance. But that may be too facile an explanation.

The gunman is from the fringe, but it is a fringe sustained politically and psychologically by a far larger and more central body of Israeli opinion. Prime Minister Begin couldn't resist twisting the knife when he deplored "this terrible sacrilege" at a place holy, as he felt it necessary to say, "to Judaism, Christianity and Islam--in that historical order."

Lebanon, meanwhile, trembles. On one side of the border sit PLO troops awaiting Israeli attack and on the other side Israeli troops wait for a Palestinian provocation. The PLO, largely immobilized in Lebanon since an American-arranged cease-fire last July, is eager for battle because sitting still means accepting an unsatisfactory status quo. The Israelis have a new reason for fighting: to try to finish the PLO militarily in Lebanon as a complement to trying to finish it politically by its village league plan in the West Bank. The tensions at the Lebanese border and those at the Dome of the Rock play back and forth. The place is a powderkeg.

Its first cause is the breakdown of sense and balance in both the Israeli and Palestinian camps. Mr. Begin plunges down a path assuring undying Palestinian enmity. The PLO makes it easy for him by continuing a policy of terror in public statements and deeds. The two sides are consummating the ultimate mutual self-fulfilling prophecy.

The second cause is the virtually complete abdication of responsibility by the Reagan administration. It is not simply that the United States, by virtue of its friendship and support for Israel, is unavoidably associated with the fruits of Israeli policy. The administration has also yet to develop an integrated strategy to defuse the Israeli-Palestinian dispute, the single greatest source of danger to American regional interests. There is always an excuse to go slow--currently, it is to allow Israel's return of the rest of Sinai to Egypt on schedule on April 25. But that deadline, assuming it's met, will leave the administration with no excuses.