MUAMMAR QADDAFI, leader of Libya, has just reminded those who had thought he was mellowing a bit of his essential vicious and duplicitous nature. First it turned out that he'd cheated on an agreement with France for a mutual pullout of their forces from hapless Chad. American intelligence discovered, and the French subsequently confirmed, that Col. Qaddafi had not removed all of the troops that French President Francois Mitterrand, after meeting with the Colonel on Crete last Thursday, had publicly given him credit for removing.

The exposure of Libya's duplicity and of France's gullibility is a matter of no small embarrassment to President Mitterrand. He now must decide whether to make good on his government's pledge not to allow Libya unilateral military advantage in Chad. The episode will, we trust, be watched closely by those several other European nations, West Germany among them, that had cited an alleged tentative changing of Col. Qaddafi's nature as justification for proceeding to expand and improve relations with the Libyan fanatic.

But that was not all. Last Friday the Colonel broadcast a defiant announcement that his assasssination squads had resumed their attacks on opposition leaders abroad. He claimed specific credit for "executing" a Libyan political exile in Cairo last Monday at 3 p.m. The Egyptians promptly produced the exile alive: their intelligence had foiled the plot and President Hosni Mubarak had seen a good opportunty to set up his foe and to let him make a complete fool of himself. Mr. Mubarak said he had information that Col. Qaddafi has planned other murders abroad, as well as other incidents in Egypt. The Egyptian president is a sober man and he deserves to be listened to closely.

Why is it that someone who has spent 15 years validating a reputation as a murderer and an agent of violent change still has a certain residual status on the international scene? The answer is plain enough: Col. Qaddafi has lots of money, a revolutionary Islamic cachet, and a talent for taunting his antagonists, including the United States, and making them sometimes look heavy-handed for playing elephant to his gnat. That much must be given to him.

The advice is sometimes offered, especially to the United States, to ignore Col. Qaddafi, to deny him a foil, as though he would then somehow dry up and fade away. But that has not happened. Others, who want to deal with him, insist that he can be tamed and civilized by being drawn into mutually beneficial contacts and relationships. That is why he was received last week by President Mitterrand, as well as by the Greek prime minister, Andreas Papandreou, his leading Western apologist. They are both considerably better situated than they were just a few days ago to report -- if they have the courage for the task -- on the value of Col. Qaddafi's word.