SO COMPLICATED is the situation in southern Lebanon, where Palestinian guerrillas, Lebanese militia, Syrian forces, Israeli troops and U.N. peace-keepers all contend, that it is easy to throw up one's hands when a violent incident occurs and distribute blame indiscriminately all around. "Cycle of violence" is the usual culprit. But that is a lazy and dangerous way to go about approaching this troubled corner of the Middle East. It permits the guilty party to get away with murder. That is why it is necessary to assess the incident in which three Nigerian members of the U.N. peace-keeping force were killed and 20 other soldiers wounded.

They were shelled by the Christian militia forces of Major Saad Haddad, whom the Israelis set up in a buffer along their northern border when they ended their invasion of Lebanon -- an invasion undertaken to counter Palestinian raids -- in 1978. But why was Major Haddad firing? Against Palestinian guerrillas, ostensibly the enemy? No. He was firing because the government of Lebanon had moved a platoon of its own soldiers into the village where the U.N. unit was stationed. Major Haddad evidently feels -- and rightly so -- that if the Lebanese army reasserts its control in the south, his "Free Lebanon" is doomed.

In the U.N. Security Council, the usual Arab-Communist combine started grinding out a resolution condemning not only Major Haddad but his Israeli patrons. The Reagan administration allowed and joined -- and anticipated -- a condemnation of the Haddad action but, seeking "balance," prevented censure, or even mention, of Israel. This is fine. The U.N. majority long ago surrendered its claim to be a forum to which rights and wrongs of matters involving Israel deserve to be submitted.

Those who appreciate seeing the abuse of the United Nations diminished, however, have an obligation not to let the Israelis get away scot free when they are at fault. And in this instance, they are at fault in not asserting control over Major Haddad, who, despite his and Israel's pretenses, is not a sovereign but simple a buccaneer. The Israelis have a right to defend themselves against Palestinian terrorists, but they have no right to let a hired gun conduct an arrogant independent policy against the U.N. peace-keeping force and the Lebanese government alike.